We exclusively use Freedom Breeder racks for all of our ball pythons. Our adults are kept in the following sizes:

http://freedombreeder.com/cage.php?cageID=3
http://freedombreeder.com/cage.php?cageID=4
http://freedombreeder.com/cage.php?cageID=5
http://freedombreeder.com/cage.php?cageID=7

We basically let the snakes tell us what they want - i.e. what makes them feel the most secure in terms of the size box they're in.

K