What size rat is okay to breed?

Yes I only have 12 rats. I'm just starting my colony up. Look, you may produce tons of rats every week, and you have every right to raise and breed them as you see fit, but many people like to wait until their rats are more mature before breeding them. There is nothing wrong with this. I personally do not like to breed any animal young or too small. I didn't say you were wrong in what you do. For all I know, breeding rats at a really young age is BEST for them, but I personally like to wait. I was simply asking why we are so worried about the problems that MAY arise with breeding reptiles too young (and even dogs and cats too), but with rodents its okay to breed so young?

okay, stay on topic here. Are you worried about age or weight?

Once you start breeding a lot of rats, you'll know when they are ready just by looking at them. It really isn't that long before they are ready to put in with the males.

I prefer to breed from large lines of females, and I hold back only females from lines that produce high numbers. We follow this so close now that most of my small (genetically, just a smaller size) rats are very few and far between.

Once you get a colony really rolling, just having some common sense will go a long way. Hold back good lines, get rid of ones with problems.