A mouse pinky is absolutely too small. I think ideally you want to be feeding a rat fuzzy or a mouse hopper. A rat pup is most likely going to be too big depending on how the place you get your stuff from measures. I don't really go by the weight myself. I feed something at or smaller than the thickest part of the snake. I have a freezer full of stuff at different sizes so I just kind of pick what fits. If you don't have a lot of animals this doesn't really work though so eye ball them at the store.