The absolute minimum is 2.5 galls/10 litres. But bigger is better and most people regard 5 galls/20 litre to be the smallest a betta should be kept in.
There is some very good info on bettas here http://www.fishforums.net/threads/betta-splendens-caresheet.423364/
There is some very good info on bettas here http://www.fishforums.net/threads/betta-splendens-caresheet.423364/
