My bettas live in spaces as large as 15g or as small as 1g, but never smaller. The average size of my bettas' habitats, not including the 15g, about 2g.
If you'd like to know, I keep my bettas at between 78-80 degrees. The pH in one tank is 6.8 (10g), and the other tank is 7.2 (15g). I really don't know what my Bowl bettas are kept in, but the pH from my tap is 7.4, which is alright, so that is what I assume they are in. Their tanks are change 100% once a week and 75% once a week, so I'm sure the condition of tape water is what they are used to. The general hardness GH from my tap is at 11, 3 in the 10g, and 5 for the 15g. I inject CO2 in the 10g and 15g for my live plants, which push the pH down rather nicely.
In Miami, FL, I've kept bettas (not spawned) successfully at a pH of 8.2, but that water is pure liquid rock and from the tap only really well-suited to some African cichlids and other types of fish that are either suited to those conditions or acclimated to live in S. FL. When I fudge with it, ei, letting it stand for a few days or so before I use it, the pH comes down to about 7.4 and is much easier to work with. The GH is still rather high, but with time, I have been able to keep demanding species such as cardinal tetras.