Usually 5-10 gal. tank will do although people have used smaller tanks. The bigger tanks allow the female a bit more room to run away in so, theoretically, there will be less damage done to either fish. I use 5 gallon tanks which are big enough to fill it only halfway, but big enough so I can do a quick cleaning when necessary without disturbing the fish.
Before you do a breeding, read up on how to breed bettas and what you will need to care for the babies. It's too late to try and get live baby food together after the babies are already born. www.bettasrus.com, www.bcbettas.com, www.classicbettas.com, www.bettatalk.com--are only some of the bettas sites I can think off that have pages on breeding bettas. Be ready to commit your life to 4 months of baby tending though. And have a plan about what to do with the babies once they grow up--bettas can have hundreds (literally) of babies at one go and if they al live to adulthood, that's a whole lot of fish to keep (boys have to be separated into individual containers at adolescence or they start biting each other).
