Ditto Kevin as far as the 5g goes.
And, make sure you don't put any male bettas together - they'll fight till death. Bettas are called 'Siamese Fighting Fish,' because of this. Females, either, unless you're absolutely sure you know how to breed them, and have a large enough tank. If you do upgrade and want to put another fish in the tank, make sure that it is 'compatible' with the betta. I don't know exactly which fish are okay to put with them, but I know that guppies are a no-no, because they have pretty fins, like the betta, and the betta will feel threatened or think it is another male betta.
Welcome to your betta-keeping journey! :D 'Tis a fun one!