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.
i do not breed my rats small, they have to be big, i have a lot of snakes so i need the big litters. All my females are very large. Im always feeding off the males and keep females from the litter to raise as future breeders.
I put my groups together at 12 weeks old and they have their first litters when they are ready.
I average 13 to 15 babies on 2nd litters with some smaller litters of 8 to 10 by first time moms.