I agree with Jason's post above (except I'm a chick not a guy)
Anyhow I will say that poor care can easily shorten and or stunt a snake. Depending on the length of time it recieved such care.
We had a 23 yr old common boa that was approx 4.5 ft. Very small due to 18 years of bad husbandry. It happens.
I would recommend giving the snake a proper enclosure (probably larger then a 30 gallon, even a breeder tank). Give her alot of time to adjust, isolate her for a good long time, and give her the proper veterinary treatment she needs including, but not limited too, at least a fecal exam.
You, in the long run will not know how large she can grow untill she grows to her full capacity. Some snakes have surprised us on how fast they begin to grow and regain their stamina when given the proper care.
I would also advise not paying for the animal. All in all if you are to buy a snake please go through a qualified breeder. If you are adopting her please just remember to provide her with the best care, including her medical care needs.
Hope this helps
Sincerely
Rusty