The enclosure size has everything to do w/ the size of the snake.
Baby balls tend to stress easily so to minimize that stress, it's best to keep them in a small tight space.
And I agree with everything the other posters have stated.

If you want to save money, I recommend going with a tub set up until your snake it's grown or can at least until it's large enough to feel comfortable in a larger enclosure. That way, you don't have to keep buying tanks and upgrading them every few months/year. Tanks are expensive. Tubs are only a few dollars.

Here's my tub size list:
6 qt - for hatchlings
16 qt - 500 or less grams
28/32 qt - 2000 or less grams
41 qt - 2000 + gram adults

Using a larger enclosure is possible, but you would have to make sure the snake has lots of hides and cover. Make sure it doesn't feel too open. Like I said, BPs tend to stress easily for insecurity.
Also ball pythons are a terrestrial snake, so they don't really need much head space. Plus the extra head space in a tank makes it a little harder to heat and maintain humidity.