"Gallons" isn't going to help any in your decision. It doesn't tell us what the dimensions are: Length and width especially. Height doesn't matter much, other than making sure there's enough room for a good layer of substrate and still tall enough that a determined tort can't climb out.
Tortoises need a LOT of room, a lot more than you might expect. Russian Tortoises are among the smallest torts commonly available in captivity, maxing out at around 6" for males and 8-10" for females. They're also among the best beginner torts... But they still need a great deal of space more than their small size would imply.
When it comes to the smaller species of tort (i.e. under 8"), my favorite rule of thumb is 2 square feet per 1" of Straight Carapace (top shell) Length. As an example, I used 6x2 ft enclosures (12 sq ft) for my 5.5" and 6.5" Russian tortoises (housed singly, mind you). And bigger is
still better!
Just with what little information you've already provided, I would recommend looking strongly into Russian Tortoises. Keep in mind, this is just one suggestion. There are several others that may do well in the enclosure you're planning, and/or just might strike your fancy more than a Russian does.
Here's a good resource for Russian tortoise care:
http://russiantortoise.org
And for tortoises in general, I couldn't recommend
http://TortoiseForum.org more. It's a great community of tortoise keepers, and is the single most up-to-date source of information for tortoise care available... Which is definitely a bonus, because it seems as though we learn new things about proper tortoise care every day: their care isn't nearly as honed in as, say, Bearded Dragons and most snakes.