I would feed it about a 80-100 gram rat every 10 days. Depending on where you buy your rats this size can be on the high end of the small rats or the low end of the medium rats. A digital scale for weighing the rats can help in getting the right size. Alot of pet stores have scales that they use to weigh their dogs and cats, you might have to convert grams into ounces though. If I remember correctly it is 3-3.5 ounces...