Wow. I feel honored your asking ME a question.
The limit for them is a ten gallon. There is a better success rate for dwarves in smaller tanks. Probably because of food reasons, since you usually have to direct feed them it's harder to find them in larger tanks. You'll often hear people say 1-10 gallon tank for dwarves. (although 1 gallon should be for experts only) if you did ever have to use a 15/20 gallon people suggest using a tank divider,