Hey! I have a 1 gallon tank, but I have to change the water a lot. Also, I have a live plant in the tank, and it helps keep it clean because it attaches the betta's waste to its roots.
I change 25% of the water at least 3 times a week, sometimes more.
I don't have a heater or a filter (which is bad), but I live in a dorm room where the room is always hot, and the thermometer that I have for the tank says its between 78 and 82 degrees. I also have a tank light which may also heat up the tank a little.
I would get a bigger tank if I could, but because of the lack of space in my dorm, I can't get anything bigger.
My betta seems very happy, so at minimum 1 gallon