And this is the exact reason we weigh our prey items. Most people don't know the difference and even fewer pet store employees know the difference. What one place calls a small rat might be a pinky rat (5-10g) and another place might consider a weaned rat (30-50g) to be a small rat. Kind of a pain, right?
FWIW, 14-21 day old rat "pups" normally weigh between 20-30 grams, which are roughly the size of medium to large adult mice. These would be suitable for a ball python that is roughly 200g in weight.