My two bettas, Bob and Boris, have a 5 gall tank each. They use every
single part of the tank. I couldn't imagine putting them in anything
smaller. Bob shares his tank with a stunted Platy that I haven't got round
to putting in the main Community Tank. Boris is on his own. I might add
some Cories at some point but I actually like the beauty of watching my
Bettas stretch out and swim in their own little worlds with plenty of
space to do so :-) - they seem to like it too

A ten-gallon with a mesh divider works well too. Both bettas have a flare
buddy and plenty of room to avoid each other when they wish. I never keep a
betta in less than half-a-ten, and they are quite happy to have a full ten
to themselves.

--
John Goulden