I just bumped one of mine up to a small rat and she is 540g empty. I was giving her weaned but she was done and looking for more after 15 minutes. She was looking like she was going to give me a bad Yelp review for small portions lol. Didn't see her for like 4 days after her 1st small. She's good. Just follow the rule of thumb, only just a little wider than the snake's widest part. I say that because I have seen a variety of what a breeder/pet store will call a "small" as opposed to a medium.