The general rule (which is more of a lose guideline) is 1 square foot of floor space per foot of snake. A hatchling BP will be somewhere between 1-2 feet long so a 10-20 gallon long tank will work really well. Cage height really doesn't mater for BP's as they are ground dwelling snakes. My adult BP cages are only 12 inches high and that's more than enough.

After that it just depends how fast your BP grows.