There is no correct size for breeding rodents, they will breed when they are mature. I have some rat females that breed very early and some when they are bigger/older. The only difference is the females that are smaller have smaller litters. I have only had one rodent die after giving birth and it was a mouse, happened to be the biggest female I had so that pretty much rules out a higher mortality rate for younger/smaller mothers. I dont know what everyone elses experiences are but I would say for myself that on average the younger smaller females tend to be better mothers.