What is the smallest tank you can breed a betta fish in?

The smallest tank you can breed a betta in has to be at least 5 gallons. 10 gallons is the best though :)

Beta's have been breed in smaller tanks, though. My moms betas bred in a three gallon tank, even though she wasn't intending to breed them. However, out of that bach, only three fry survived.

Also, you don't want to fill the tank all the way. Spawning tanks should be filled to only three to five inches. It makes it easier on the male to keep the fry in the bubble nest, and it can make spawning itself quicker on the male and female.