IMO i like to use a 40gal. breeder...
I don't really use tanks anymore, but hen i did, i tricked em out with big water bowls, a few hides, and tons of plants.
but a 20 gal. would be great too.
Many ball pythons would not eat in enclosures that large.
20 gallon tanks would suit an adult ball python for it's entire life IMO.