congrats on your first snake and picking a ball python! a tank or enclosure of that size should fit your ball just fine, some things to think about though.

are you buying a hatchling or a juvi or an adult? a hatchling usually can live in a 10 gallon aquarium for a pretty good stretch of it's life. as they grow, ball pythons will generally need more length and width then height. a 20 gallon long would be acceptable ( I believe they are 12w x 30L x 12h) but something like a 40g breeder with a few good hide boxes would also work. Something of that size should be perfectly fine for your snake as it grows.

you don't want to go toooo big because it stresses the snake out (as I am sure you have read up on) but two small is harmful as well! something of the size you are referring to should work out just fine for them may want to beef up that 8.5 to about a 12" just to be sure good luck and happy herping!

CONGRATS!