The smallest rat I've produced was about 80 grams at weaning, and most pet store rats are going to be considerably bigger. Most "small" rats I've purchased from pet stores have been around 130g, with the very smallest probably somewhere around 100, but that was a very rare find. Honestly, I'd rather go too small than too big here and just go with the biggest mouse they've got (mice average out around 20g as adults).