That tank size sounds fine for one that's a yearling or a bit older. Mines a 36x12x12 and mine is a bit longer then that aquarium. I believe in giving them some space, but also clutter up the space with fake plants, make they have 2 hides and so on. If they have too much room they can get stressed out and not eat etc.

Just to give you an example of how fast they grow, when I first got mine, the 2 hides I had for her I thought would do great for atleast a year, 3 months later she still likes going in them, but I can always tell which 1 shes in because the hide is lifted off of the ground. yesterday her back was sticking out of the backside of the hide as well.

Thanks