what dimensions of the enclosure will i need...

you would be fine with a 40.

I'm no expert but i do know that a 70gal is way too big.

Ball Pythons spend there lives in the wild in the cramped spaces of rodent burrows so bigger cages cause stress.

I keep my bp in a 20gal long which seems ideal. He has 2 hides, and a fake vine to make it feel crowded. Also 2 water bowls to keep up humidity. Anything bigger than a 20gal long needs lots of hides and plants to keep your bp feeling cozy.

But I wouldn't get too much bigger cause like I said, a larger enclosure is harder to maintain. It's heavy and hard to clean, costs more to replace bedding and such, takes a lot more work to keep humidity and temps right, and you have to buy more hides and stuff.