Additionally--we would like to get one that stays small (5-8 inches or less) when fully grown.  I'm not even sure this is possible--I'm just throwing it out there as an 'ideal'.  Thanks, guys!

there isn't anything that i can think of that will live happily in a 10 gallon... i believe russian tortoises stay smaller, but they still need much more space than that.  I do ot know too much about turtles or tortoises, but i do know they all pretty much need a lot of space.

You are not going to find an aquatic or terrestrial turtle that will stay in a 10 gallon aquarium forever (some hatchlings are good for them though).  If you're looking into a terrestrial species, I would suggest a Russian tortoise.  They stay within the size range you want and are fairly easy to care for.  They have special diet requirements though, so research all of those.  They also never stop eating, my Russian that I had was a powerhouse feeder (mostly dark, leafy greens), so be prepared to buy a lot of collards, swiss chard, kale etc.  Research the animal in-and-out before buying it.  They will need a fairly large enclosure as an adult, turtle boxes work well and are easily assembled (as this species isn't the best climber).  4' x 4' would be great for one tortoise.

Cheers!

+1 Geegmasta. There is no turtle or tortoise that will be happy in a 10 gallon. You might be able to get away with it as a starter tank for a smaller species hatchling, but that's about it.

As has been suggested, Russians are great tortoises that stay small, with males usually under 6" and females usually under 8" (though I've heard older females can get up to 10", it seems to be rather few and far between in captivity). So are Hermann's and Greek tortoises, both staying in about the same size range as Russians. North American Box turtles are also very small--not usually over 6", if I'm remembering correctly. Any of these still needs a large enclosure.. Tortoises and turtles are a lot more active than people like to think! A 4x2 ft enclosure would be bare minimum, in my humble opinion, for any of these, with ideal being the largest you can provide (plus a nice outdoor enclosure for the warm weather months). 4x4 ft or, if you can't fit something quite that wide, 6x2 ft are a couple good sizes to start at for an adult of one of these species.

If you decide you want to go aquatic, some of the smallest (again, if I'm remembering right; aquatics have never been my forte) are the North American mud turtles and musk turtles, with some staying as small as 4-5" as adults. These guys are small enough that you can get away with a 40 gallon Breeder tank for an adult (3 ft tank), though bigger is, as always, better. There may be other similarly small aquatics; I just don't know which ones they are.

RESEARCH, RESEARCH, RESEARCH! Naturally, research is necessary no matter what reptile you're getting, but with tortoises and turtles it's especially vital. People have a habit of vastly underestimating what it takes to properly care for a turtle or tortoise (thinking one would fit in a 10 gallon is a prime example of this). http://tortoiseforum.org is a great starting place for learning tortoise and turtle care, and http://russiantortoise.net is a great place to start for Russians, specifically.

And those tiny turtles in the 5 gallon tanks at expos that claim "They will ALWAYS be this small"...is a lie.

You could get a 55 gallon aquarium and set it up for a stinkpot turtle. They are one of the smallest species of mostly aquatic turtle that you'll find.

The only turtle that I think qualifies for a 10 gallon would be a stinkpot musk turtle, or something similiar and in the same family (Kinosternidae). They stay small. However, if you can go larger than a 10 gallon, like a 20 gallon long, please go ahead and do it. More space is always better, especially when it comes to turtles.

Web-- JMHO... Stinkpots and their relatives may be small, but I still do not believe an adult would be able to live comfortably in a 10 gallon tank. I don't know your experience with turtles, so maybe you just haven't kept any and been able to witness how active they are... But, truly, a 10 gallon tank for an aquatic turtle over a couple inches SCL (maximum) simply isn't adequate.

Look at the mud and musk turtles. They stay relatively small. A male three striped mud doesn't get much bigger than 3" and while a 2' long tank would be better, a 10 might work... there's not too much difference in space between a 10 and 20. Go 20, and give a mud turtle a chance. They are very personable.

Keeping any kind of tortoise or turtle in a ten gallon should only be for a hatchling, since they grow fairly quick under optimum conditions you may want to secure a second future home for your pets or pet. I have kept all kinds of turtles and tortoises and find plastic totes are easy to clean move and find, and much cheaper than glass or frabicated enclosures. They also come in various sizes, can have heat mats under or on the sides and hold humidity very well. They can also be filled with water and filtrated for aquatics. Mud turtles stay small ,are adorable and are aquatic and they are hardy...

Get yourself a much larger enclosure if you plan to keep any type of turtle or tortoise, and buy some guppies for the 10 gallon.

Hey I just met you. And this is crazy. But award some points. Close this question, maybe?