7 yrs old and three feet??? That sound an awful small for that age... even for a female. That iguana must be extremely malnourished and with possibly MBD to be that small at that age... heck, it probably could not have reached that age under such poor condition!
Chances are it is younger than people told you.
Anyways, reptiles in general never cease to grow. They grow 'til the day they die. Of course, with age the growth increases with fractions.
Once the iguana checks out ok, and is brought back to ideal captive and living conditions, it should regain some size and length. IF it is true that the animal is 7 years old... it'll grow very little, even if fed well, because of its age. However, if it is actually younger than told, cances are it might gain a foot after a year or two.
Good luck with it,
-IJ