I have all sorts of tanks ranging in size from 1 gallon- 20gallons. I think 1.5-2 gallons is smallest Id go long term for an adult betta. I guess it depends on the bettas size . My Petco bettas I got as babies are on the smaller side. I have tried a betta in a 20 but he didn’t live long but I suspect he was ill. It seemed like a lot of tank for him. I have a Koi betta in a 10 seems to be working for him. If I get in a jam (which usually means I bought another fish lol) I have a 2 gallon crystal wine/ice bucket I use lol. I don’t notice too much of a difference between my 1.5, 2, and 3 in terms of water quality but cycled and I change water very often and use Prime. I need to take some new photos of my many tanks, maybe this weekend !