Is it possible to predict from a baby's birth weight and length how big an adult he will be?
Absolutely not. Any pediatrician can tell about the tiny preemies who grow into basketball player-size men, or the giant babies who turn into average-size adults. The reason pediatricians keep close tabs on a baby's height and weight isn't to predict the future, but to make sure the child is developing properly, whatever his initial -- and final -- size. Doctors are less concerned with the specific height and weight of a child, and more concerned with the pattern of continued growth.
The growth charts that pediatricians consult compare a child's size with the general population. So a child who falls into the 50th percentile for height and weight will probably continue to follow that growth curve for his first year or so. But some children jump their curve; they may start out life in the 50th percentile for size, but then grow more quickly than average and jump to the 95th. That may be when genes come into play, and the small child of large parents starts to grow into his genetic heritage. Assuming that a child is in good health -- and his growth isn't compromised by illness or poor diet -- his final size will have far more to do with his parents' size than his birth weight.
A very rough guideline is that if you double a child's height at age two you will have an approximation of his eventual adult height. But there are many exceptions to this rule.
Sigh. Curtis is going to be a shorty...unless he really sprouts in the next two months. :P But, I did have to raise his basketball hoop again last night. :)