Hmmm... I would like to promote a minimum 1 gal size, but realistically, a 3/4gal jar or even a 1/2gal would work too.
It is not so much a size issue for the juvies, not for the first few months anyway, but IMO it is a water quality issue. Anything under 1gal should be cleaned twice a week, so it means more work.
My juvies will end-up in a custom betta barracks tank system I designed. Each get about 1.5 gallon of water, an airstone and foam custom filter, and weekly partial water changes, with a monthly complete water change.
I need to cut about 450 pieces of glass, and have a lot of caulking and airlines to make, but I am hoping to sell/consign my barracks with my bettas to LFS owners. The idea being, buy my fish, use my barracks to display them (versus cups) and everyone wins!