Or you could increase the air flow around the tub with a fan. It worked for me.
By buying a tank, you're going to have to keep fueling it with money to get it to work. I think they're too much trouble.
But if you're insistent, go with the 20g. You'll never need a 40g, as bp's love tight spaces. Snakes don't care what their in, as long as their husbandry requirements are met. Extra stuff is for our satisfaction only, not theirs.