Great Dane dogs generally grow till about 15-16 mths. Though this is not a hard and fast rule as some have continued growing till abt 2 and a half to 3 yrs too and some stop a bit early. Bitches are generallly short and i believe you already have exercised on these facts. I all depends on the genetic predisposition man. If you have seen her father and mother rest assure she will be fine. Moreover if you have got her as a pet, size doesn't matter.
My bitch is 34 inches at the shoulder and she was growing tall till abt 15 mths of age.
Do post some pics of her. You can see mine in my profile.