I've got my boys in 10 gal divided tanks. The smallest I've ever kept was my first betta in a 5 gal. I will never keep anything smaller than a 5 gal because it is hard to hold a cycle. I love my once weekly water changes.
The minimum I have right now is my two hospital tanks, a 0.25g (0.5g filled halfway) and a 1.5g for my perma floater Kris. But as for healthy Bettas, 2.65g and 3.3g is what I have for the other 8 males.
Ghoul (male VT) lives alone in a 3g tank. Lich (VT) shares an 8g tank with 5 young Cory panda and 2 young apple snails (I know, it's a bit crowded but I plan on upgrading to a 15g really soon). Baal/Andariel (still not sure if male or female, I have a thread about that) shares a 3.5g with an apple snail.