CornCrazy's right.
So far, I only have one adult that is big out of all my children. It would be cruel for me to put him into a 20 gal tank. I keep him in a home-built cage that is 18 inches square by 48 inches long, w/ a glass front, and sometimes, I feel that that is too small for him--mostly when he's moving around inside it.
Oh yeah, one more thing: snakes never stop growing, although they do slow down once they're adults.