How will I know when to move up from rat pups to something bigger?
if you're unsure, smaller prey more often is better than risking an excessively large meal. however, the best advice is just to trust your instinct. but if you're looking for a general informational guide to feeder sizes, the following thread was just posted today. I'd recommend giving a look at Justin Kobylka's youtube video that I linked to as well.
As it was said before, once you stop seeing a noticeable lump in the snake after it eats. You want the prey item to be about the same size or a tad bigger than the thickest part of our snake.