I am no pro.... yet, but what I've learned from others so far is that the size is not as important as the maturity of the female. Most breeders refuse to breed a female before she is atleast 3 years old, 6 feet in length. The size is not as important, it will just have an effect on the size of the litter. More importantly than the length of the snake, is thickness. I do not mean how fat as she, it should be healthy thickness, "muscle" I guess. If she does not seem overweight, but it only 5.5", most would say go ahead and breed her.
Another thing to take into consideration. if she is a virgin, he first litter will typically give less babies. I can't say this is always the case, but it is not uncommon for a young female's first litter to do so.