Most people use at least a 1 gallon container, As for the heat!! Betta splendens can live in water from 72-85 degrees, The 85 being breeding temps. But as long as your house dont get to cold or to hot, And stays some where around them temps then your fine.
And most of the time we even go as far as building a betta drip system so that they do have heated water and fresh filtered water.. I hate tanks without a flter or some source of water movement via air stone or something. The water gets nasty fast and then the fish gets shocked with all new water once a day, If the people keep up with the daily changes... Its just not my style, I love sponge filters, the best thing since slice bread... Just a air pump and a sponge..
And then you build a sump and you have a drip system so the fish has fresh water 24/7
I mean bettas can live in a coffee cup, But will they thrive in the coffee cup?
They thrive in bigger aquariums, They thrive with clean filtered water.. With a nice planted tank... But they just live in a little bowl, Most if not all the time they end up sick or dying from the little containers.... From not having a filter on the tank, people dont realize how important fresh water is everyday if there in a Non filtered bowl/ container....