Probably not. Once you add 3 or 4 inches of substrate, your 6" sides would only be about 2 or 3" tall. A Russian can climb that easily. In my opinion, the minimum size for a full grown Russian tortoise would be about 2' x 4 or 5'. And bigger would be better. Can you set the tortoise up outside?
Oh...sorry, I mis-read that. I see the sides are taller than 6". But the size still is too small in my opinion.
I would really go bigger, that looks small :/ If you can try building a outdoor haven for them like Emysemys said.Also I did see a rock water bowl, most tort owners use a potting plant bish its easyer for the tort to go inside and soak and bigger, more round.
Thanks for asking our opinion! were just trying to do the best for your russian
I'd prefer to go bigger too. And those water bowls are death traps. Not too long ago a member had their tortoise accidentally flip over and drown in one of those. I'd replace it with a terra cotta plant saucer from a hardware store. Those only cost two or three bucks. I'd get two and use one as a feeding bowl.
I would say absolute minimum for a full grown russian would be 4' by 2', however there is no such thing as too big. Personally I would get an old bookcase (you can find them at thrift stores for $20) lay it on its back and removes the shelves, line it with a tarp or something similar, and use that as an enclosure for inside. Outside is definitely better though.
I've always heard 2 feet by 4 feet for a minimum as well. In all honesty, your 2.5 feet by 3 feet is not really a whole lot smaller than 2 x 4. However, as others have said the bigger the better usually.
A friend of mine had an outdoor enclosure for his Russian that was a third of an acre, and that tortoise still did all it could to get out, so bigger is always better.