A six inch telescope is pretty much the amateur astronomers entry level size telescope
This size telesocpe is a good size for beginners to the hobby of astronomy. You get some good light gathering power at a very reasonable cost. And, if you are a bit more serious about astronomy, or already have a telescope and you want to upgrade the six inch size also offers you some good choices. Let me explain further.
In the six inch size of a telescope you have three general types that you can choose from and which one you choose will depend on quality of the telescope, seriousness of the hobby, and amount of money you want to spend.
Here are the three types (priced lowest to highest) with a quick synopsis:
1. Dobsonian Mount - This is a terrific entry level telescope because you get a lot of light gathering power for a very reasonable price. You make some sacrifices in that you won't have the ability to auto track, or auto find objects and the simple newtonian set up is just a basic set up -but works well. And it is an absolute bargain at dobsonian prices.

Orion SkyQuest XT6 Classic Dobsonian Telescope
It's no wonder customers heap 5-star ratings on this "gentle giant." For not only does its jumbo-sized optics and uncomplicated design bring a "new level of joy to simple observing," raved Astronomy magazine, but it's also one of the most affordable quality Dobs on the market. The SkyQuest XT6 Classic isn't just good bang for the buck, it's a supernova of telescope value! The XT6 Classic gives you the deep-space thrills without the deep-pocket frills. We've kept it lean and mean to keep its price low for tight budgets. But rest assured, it comes fully equipped for adventure, whether you're a beginning stargazer or are graduating to a more capable instrument. It features an expertly figured parabolic mirror housed in an enameled steel optical tube. The tube rides on a stable Dobsonian base that allows easy point-and-view navigation and has a convenient carrying handle. A 2" Crayford focuser, EZ Finder II aiming device, 25mm Sirius Plossl eyepiece (1.25"), and quick-collimation cap are all standard equipment. Setup takes only a minute, leaving the rest of the evening to marvel at the planets, the Moon, and a myriad of deep-sky treasures. Enjoy the fantastic views - and savings! One-year limited warranty.
2. Newtonian/Equatorial Mount- This type of telescope has the same tube configuration as the dobsonian. The difference with this type of scope is the mount. Instead of being on a dobsonian mount this type of telescope is on an equatorial mount. It makes a difference in how you use the telescope and in your ability to follow objects across the sky. It also takes a bit of time to get used to this kind of mount. If you are not familiar by what an equatorial mount means it is a mount that you align with the north or south pole (easy to do) and then the telescope is very easily rotated to follow objects. It counteracts the motion of the earth. Absolutely necessary if you want to take pictures. And many of these telescopes come with an optional clock drive which means you don't even have to move it. It will automatically track whatever you point it at.

Orion AstroView 6 EQ Reflector Telescope
If you're ready to graduate from a small scope or want your first scope to have more light-gathering power than typical starter scopes, step up to the AstroView 6. Its 150mm parabolic mirror has a focal length of 750mm (f/5), so it offers generous wide-field views of deep-sky objects when used with the included 25mm Plossl. For lunar and planetary study, use the 10mm Plossl (also included). The AstroView's EQ-3 equatorial mount features manual slow motion controls, which allow you to center and track objects in the viewing field. An optional single- or dual-axis EQ-3 DC drive is also available for electronic guiding. Built in to the equatorial head is a polar-alignment scope for precise orientation on celestial north or south. The mount couples to a sturdy adjustable aluminum tripod with accessory tray. Includes 6x30 finder, rack-and-pinion focuser, and four-vane adjustable secondary mirror support.
3. Schmidt-Cassegrain - This type of telescope is pretty much without compare. If you want superb optics that will get the most out of your telescope then this is the type to get. You will be very pleased with the performance and the options. But, this of course will come at a price. These telescopes are typically at least twice the cost of the dobsonian or equatorial. Because it has folded optics it is a small telescope which makes it much more portable than the other types.

Celestron NexStar 6 SE Telescope
Portability and serious performance come together in the NexStar 6 SE. Celestron's first new Schmidt-Cassegrain optical system in over a decade is now available in a high performance altazimuth computerized telescope. With a total weight of 30 pounds including the tripod, the ultra portable 6 SE features a precision optical system with 1500mm focal length (f10) and offers 44% more light gathering than a 5-inch telescope.
The NexStar 6 SE has all the same features as Celestron's most advanced computerized GoTo telescopes, including our revolutionary SkyAlign alignment technology, a sky tour feature, a database of over 40,000 celestial objects, and easy-to-use hand control.
Some Popular and trusted Makers of telescopes in the six inch range
I recommennd you stick with a known and trusted manufacturer of telescopes. Here are some more six inch options for you to look at.

Orion StarBlast 6 Astro Telescope
We've done our popular Orion StarBlast 4.5 Astro Beginner Telescope one better. The Orion-designed StarBlast 6 Astro Telescope takes the same compact, wide-field, user-friendly concept and expands on it, literally. The StarBlast 6 boasts fully 73% more light-gathering area than its smaller counterpart. Yet the scope is still highly portable, weighing only 23.5 lbs., and comes with the base fully assembled in the box! The StarBlast 6's 150mm (5.9") f/5.0 parabolic mirror provides ample aperture to reveal the planets and deep-sky gems in wonderful detail. Its wide field of view makes finding and tracking objects easier than with longer focal-length telescopes, which makes it a superb beginner telescope -- for all ages. It also would make a terrific "grab-and-go" second telescope for more experienced stargazers. Teflon bearings and adjustable altitude tension on the base ensure smooth maneuvering of the StarBlast 6. Keep the included Sirius Plossl eyepieces at the ready in the convenient three-hole eyepiece rack. Two handle cutouts in the base allow easy lifting and transporting of the telescope. Set it on the ground or even on a small table or the hood of your car, and enjoy the view! The Orion StarBlast 6 isn't just a masterfully engineered, uncomplicated, and affordable telescope for all ages. It's also a blast! One-year limited warranty.

TwinStar White 6
This telescope is ideal for backyard astronomy enthusiasts or older children with space on the brain. Simply turn the telescope on, choose the object you want to see on the handheld controller and the computerized GPS mount does the rest. With a large 150mm (6 inch) primary mirror and a 1400mm focal length, this large reflector telescope is the perfect high magnification instrument for planetary and deep space astronomy. You can observe fabulous nebulae, galaxies, binary star systems and most of the famed deep-space Messier objects. Of course, with this telescope, you can see outstanding detail when viewing the moon and you can see the Rings of Saturn and the Moons of Jupiter. The iOptron SmartStar-G Alt-Azimuth Mount with GPS, a.k.a. The Cube, is probably the most functional and flexible unit on the market. Both axis motors are built into a small single unit. In addition, there are no "dead spots" -- so you can point your telescope anywhere above the horizon, whereas, other mounts block the telescope tube at certain points of rotation. This mount comes standard with AC connection, but also accepts 8 AA cell batteries for use at home or in the field (Batteries sold separately). The GoToNova computerized control system is by far the most technologically advanced automated tracking system available on the market today. With a database of 50,000 celestial objects, including all of the most famous galaxies, nebulae, star clusters, not to mention the planets, you'll be able to enjoy star gazing with the simple push of a button. There is no need to align the telescope, with GPS, it knows its location. The GoToNova Controller is much easier to use than other similar products. The hand controller is more intuitive with menu categories better organized. It also has a larger LCD screen with more lines of content compared with the competition. With the easy-to-use hand controller you can easily set up your telescope and select where you want to go.

Meade ETXLSSC 6
The Meade's revolutionary ETX-LS LightSwitch telescope uses advanced technologies like GPS, LNT and ECLIPS CCD imaging to do what no other consumer telescope has done before: take all the hassle out of using a telescope. Simply flip the switch and the ETX-LS automatically aligns itself. Astronomer Inside then takes you on a guided multimedia tour of the best objects in your sky. The new aluminum mount is lightweight and portable, yet has a rigid structure with precision worm-gear drives for the ultimate in tracking and pointing accuracy. All this combined with Meade's Advanced Coma-Free optical system provides the stargazer with the most sophisticated, yet easiest to use telescope ever.

Meade LT-6, 6
The Meade LT 6" f/10 Schmidt-Cassegrain Telescope provides the backyard astronomer with a feature-filled scope at a very attractive price. The scope's standard equipment AutoStar computer is amazingly simple to set up and operate. You don't have to know Polaris from the Pleiades, or Albireo from Zubeneschamali. All you have to do is level the scope, point it North and tell the AutoStar where you are on Earth and the time and date. The AutoStar computer will do the rest, aligning the scope on the skies and letting you locate dozens of deep space objects like a seasoned observer your first night out. The simple tripod/altazimuth mount configuration of the LT-6SC makes setup fast and easy. The rapid alignment features of the AutoStar computer and its simplified menus will start you observing in only a matter of minutes. The wide array of objects in the LT-6SC database and the 5 arc minute pointing accuracy of the AutoStar computer, will accurately speed you from object to object with no frustrating hunting or star hopping. With the LT-6SC, you'll spend more of your time looking at objects and less time looking for them. If you're one of those busy people whose schedule doesn't leave you much time to enjoy astronomy, an easy to use fully-computerized LT-6SC will make the most of your limited observing opportunities.

TwinStar Blue 6
This telescope is ideal for backyard astronomy enthusiasts or older children with space on the brain. Simply turn the telescope on, choose the object you want to see on the handheld controller and the computerized GPS mount does the rest. With a large 150mm (6 inch) primary mirror and a 1400mm focal length, this large reflector telescope is the perfect high magnification instrument for planetary and deep space astronomy. You can observe fabulous nebulae, galaxies, binary star systems and most of the famed deep-space Messier objects. Of course, with this telescope, you can see outstanding detail when viewing the moon and you can see the Rings of Saturn and the Moons of Jupiter. The iOptron SmartStar-G Alt-Azimuth Mount with GPS, a.k.a. The Cube, is probably the most functional and flexible unit on the market. Both axis motors are built into a small single unit. In addition, there are no "dead spots" -- so you can point your telescope anywhere above the horizon, whereas, other mounts block the telescope tube at certain points of rotation. This mount comes standard with AC connection, but also accepts 8 AA cell batteries for use at home or in the field (Batteries sold separately). The GoToNova computerized control system is by far the most technologically advanced automated tracking system available on the market today. With a database of 50,000 celestial objects, including all of the most famous galaxies, nebulae, star clusters, not to mention the planets, you'll be able to enjoy star gazing with the simple push of a button. There is no need to align the telescope, with GPS, it knows its location. The GoToNova Controller is much easier to use than other similar products. The hand controller is more intuitive with menu categories better organized. It also has a larger LCD screen with more lines of content compared with the competition. With the easy-to-use hand controller you can easily set up your telescope and select where you want to go.
Here is A Rare Telescope
It can be a challenge to get a great quality six inch telescope in the refractor family. Here is one

Meade LXD75 AR6-AT Refracting Telescope Autostar Achromatic 6 inch 1219mm f/8.0
Includes Meade LXD75 AR6-AT 6 Inch Autostar Achromatic Refractor Telescope Heavy duty German Equatorial Mount Autostar 497 Hand Controller Smart Drive 8x50 Viewfinder 2 Inch Focuser With 1.25 Inch Adapter High Precision Pointing Heavy Duty Tripod 26MM Super Plossl 1.25 Inch Eyepiece. You want a breathtaking view of the universe? This is the telescope that will not disappoint you. The optics of this telescope are above comparison when it comes to telescopes with mirrors.