My rat seems small for her age. I think she is about 5 months. I've had her since she was 4 weeks. I have a younger female who is about 3-4 months and she is bigger. I've looked it up but can't find any answers.
I agree with Forensic - unless she shows signs of illness etc I wouldn't worry. Sometimes you just get the odd 'shrimp' of a Rattie. LOL
Eg, I have 4 sisters, Lily, Jennie, Niki and Kitty. Lily is the smallest by far; she's 1/3 smaller than Kitty (the next biggest) and Niki and Jennie (the two biggest) are perhaps double her size. Although they've all got the same genes, Lily's just the tiny trouble of the group and happy as larry
There was a male runt in Dotty's litter and he was tiny, but yet managed to survive - Until my brother had him for a few months then he passed away, but I believe that his evil girlfriend did something to him, as she kept on moaning that she didn't like him :x