For the most part, I agree with Laurenie. 2 gallon and larger containers are the most suitable because below that, they become difficult to heat. The smallest adjustable heaters are typically designed with a 2 gallon minimum in mind. I don't feel that heaters are optional at all for keeping bettas, they are tropical fish that need stable temperatures in the range of 78-83 degrees--unless you live in a tropical climate these temperatures probably are not the norm for you, and even so, there's no telling when you might encounter a cold front.
Smaller tanks are also harder on the fishkeeper from a maintenance standpoint--a 2.5G container would need a 100% water change every 4-5 days, and a 1.5 would need a 100% water change every 3 days. Larger tanks are easier to maintain a colony of beneficial bacteria in, these bacteria break down the toxic ammonia fish excrete as waste into much less harmful compounds, eliminating the need for frequent 100% water changes. With cycled tanks, you only need once weekly partial changes. It is possible to cycle small tanks, but you would need a lot of live plants and a decent filter that doesn't produce a lot of current, but has enough surface area to house a bacterial colony. This is another reason why bigger is better.