The size of rat depends slightly on genetics, but mostly on diet. A rat that isn't offered a nutritionally complete diet is not going to get big. Babies also need to be fed higher protein to grow. What kind of food are you feeding? Sadly there really isn't any food at pet stores that are acceptable for rats, unless they just came out with a new one in the last couple years. Hairless is a recessive gene, you need hairless rats or heterozygous hairless (rats that have hair but have 1 copy of the recessive mutation) to make them.