The prey item should be as big around as the widest part of the snake. A photo would probably tell us more than a description. If you think rat pups are too small, move up to the next size rat. (Usually the progression is the following: Pinky, fuzzy, crawler, hopper, small, medium, large). Crawlers are sometimes called pup rats, but I have heard fuzzies referred to that way as well. Fuzzies have their eyes still shut, and crawlers have their eyes open, but aren't very coordinated on their feet yet.