Its easier to think more in terms of weight.

An adult mouse can weigh in anywhere from 30-50 grams. Its Rat counterpart would be a "weaned" rat which runs about the same weight.

Ryan, it all amounts to what you have available to you in terms of what you should do. Most people have a hard time finding live rats in smaller sizes, others don't.

All sizes available, I'd start a hatchling on hopper mice, move them to rat fuzzies/pups after a few weeks, then go up to weaned rats, then smalls, etc etc...

Multiple adult mice at the proper interval can be fed to 400-500+ gram ball pythons without any concern.