Mr. Big is living in a 3 gallon tank (it was a kit from walmart) and he seems to enjoy it. :] I also have 3 plants in there for him, and he loves swimming around/through them and they also give him a place to sleep. And a heater so he will not get cold. Although soon I'm switiching to a split 10 gallon because I've recently also rescued Osiris :]
My sister has her single betta in a 5 gallon tank with a few fake plants, a hiding place, and a heater. He is very happy and healthy. I would highly suggest this set up as its easy to maintain, and relatively cheap :].
and ps, is that Blu in your avatar? Hes gorgeous!