I like to make sure my boy bettas get atleast 5 gallons each...In the past I kept my bettas in smaller tanks but I think my guys are more lively in 5 gallons and up and they really enjoy the swimming space.Larger tanks hold a nice cycle which means if i get busy and skip a day or two of water change my fish will be ok...It is also alot easier to find a heater for a bigger tank. I do have a blind betta in a 3 gallon though and he seems to enjoy it...so I think 3 gallons would be the smallest tank size I would get, but I like to give my guys 5 gallons or more usually..
I didn't vote because there isn't a multiple choice one lol.. I keep mine in 1-2 gallons, (mostly 2, have 1 1 gal being used at the moment) unfiltered. Simply because of the amount of room I have available for these guys without going for even more multiple large tanks. Easier to place them on shelves then give up floor space that is occupied by furniture, kid and dog hehe. I would love to have them all in larger tanks, but they are warm, clean water, planted and healthy.
I don't think there is a minimum. As long as water quality is good and they are warm, any size works. I.E. How breeders keep them. If someone has a room they keep heated and keep their bettas in a 1/2 gallon of water (or less) and do the necessary water changes, those fish are just fine as far as I am concerned.
That being said, I personally won't keep my fish in anything less than 2 gallons. It is too much work on anything smaller.
I just voted for the size I have now (sort of). I think 1 gallon and above is fine. I found a good deal on a 3.5 gallon and my boy seems to be exploring the territory, hanging out in temple (so glad he uses it), and guarding his territory in that one reflective corner. o.o;