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!

 

1.5 gallon is fine, Some people's fish would give a right fin to be in that. Because people have them in small container's.

I bought a 1.5 gallon for my little brother, It has a hood with a led light, A wisper filter and air pump. For 15.00 dollar's all i had to buy was gravle and plant's. Anyway long story short everything has been doing great for a year now. I make him do 50% water changes every week and everything is clean and brand new looking a year later. No sick fish and teaches him responsabilty.

 

I have 2 of mine in 1.5 gallon critter keepers.