20 gallon long will be perfect for a ball pythons entire life. You might need to move to something a bit larger if you have an extra huge breeder female.
Just so you know, ball pythons aren't the best display snakes so you will most likely be staring at a bunch of fake plants and no BP.
You don't know my bp, then. lol. He's all over the place.
I wouldn't keep one in anything less than a 30. Bigger is better of course. Mine is in a 50 gallon. Bps love to stretch out and given the chance, will be fairly active snakes.