You're correct that a RUB (really useful box in the UK, aka tub here in the US) is preferred over a tank for the reasons you stated. They also tend to be less expensive to set up and maintain.
Rather than get hung up on size I would focus on the following:
- Does it have handles that flip up to latch the lid securely? Some lids pop off with no effort, which means your snake can go for an unauthorized slither.
- Is it well made? Cheap ones can warp or crack easily.
For size, you can run a baby snake in a larger than ideal RUB if you provide lots of hiding places and clutter it up. You don't have to go wild with decorations either, loosely crumpled newspaper can take up extra room and make your snake feel secure.
You'll want a RUB with a decent amount of floor space compared to its height. Ball pythons don't need to climb and while they spend most of their time in hides, they should have enough room to stretch out periodically.
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