Snakes and other reptiles will continue to grow through their entire life. The most common length is between 4 and 5.5 feet in length. I have an adult snow that is 4 years old, and is about 49" right now...so a little over 4 foot. After a corn reaches his or her maximum length, they will gorw, but very very slowly. You probably won't even notice that the snake is any longer.
As far as girth, that depends on the amount a corn is fed. The said snow above is about the sice of a Half Dollar coin, and about 400 Grams.
Hope this helps.