I think you could probably use the 5 if you put a small Tupperware inside it. All I put inside it was a tiny water dish, a tiny fake plant, a tiny piece of toilet paper cardboard tube, and aspen substrate.
I just dug out the tank he started in (he came to me in it, I didn't buy it) and looking at it it might even be a 1.5 gallon may be 2. I always thought it was 2.5, but I may be wrong. It's freaking tiny though.