I would say it depends on what the fish feels more comfortable in, but larger is better. (Let's keep our minds on the topic ) So, a min of 1gal and as large as the betta can handle.
It really does depend on your fish. If you put him in a 10gal and he looks unhappy, move him to a smaller or larger tank. There really isn't a set amount/size for a betta... just what makes them happy.