Hi,
I agree with Zagz. I've had seven bettas thus far, some housed in bowls and others in tanks. My oldest is going on 3, which is a ripe old age for a betta, and has lived since the pet store in a 2.5 gallon, filtered tank. My newest is in a 10 gallon (I had planned for him to be part of a community in my 30 but he didn't get along). The others have been in jars, all at least a gallon, a couple a bit larger, and all of them, despite scrupulous water changes, careful efforts to feed only "just enough," and etc., eventually succumbed to various ailments.
You can get a 5 gallon at WalMart for about $35, and I think it's worth the investment. 2.5 gallon aquabows would be another option, but the smaller the tank the more crucial it is to stick to a strict schedule of large water changes at least once a week.
As for the spot, it does sound like it's probably a fungus, but a photo would be helpful. My suggestion would be get the 5 gallon tank, and put the betta in, along with a plant or decoration from his bowl to help start the bacteria cycle. Otherwise leave it bare for the time being, while the fish is being medicated. Later you can move him to a bowl briefly while you decorate the tank.
L