1.5 would work as long as you kept up with the 100% water changes (every 2-3 days, not longer). Think about the fact that she will be swimming in her own toilet, essentially. Water changes will need to be made quite often in a small tank like that to keep her waste buildup down. The bigger, the better. I'd recommend at least a 2.5 gallon, but better would be a 5. This would allow you to be able to cycle the tank, thus doing water changes less often (and no 100% changes), and put less stress on her (hardly any waste buildup and not changing the tank around 100% every 2 or 3 days).
I hope this helps!