A 10 gallon is a great way to start. A Ball Python can live in one for several months. I've got a 10 and a 20 gal. If you're getting a baby snake then there will be a lot of extra room in even a 10 gal. When a BP has too much room it can lead to stress and not eating. You've got to clutter up the tank a lot with plants and at least 2 hides. Here's my 10 and 20 for example.

10 gal


20 gal


Hopes this helps