Yeah, that "rule" is a good way to determine minimum enclosure size, and you can always go larger provided there is adequate cover and visual barriers to make the snakes feel secure.
I agree with Deborah that floor space is the most important consideration, but despite being classified as terrestrial wild ball pythons do show some tendency to climb. Particularly juveniles and smaller males. You can build a more enriching environment in a tub with a little bit of height in addition to the floor space.
Edited to add that, whatever size you end up choosing, tubs and PVC enclosures are always superior options to tanks when it comes to ball pythons. Much easier to set up and maintain.
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