How big will my 6mth old filly grow?

As a general rule a foal will be shorter than the sire when coming from a shorter mare, you can some times go by the string, but you never know for sure whether it will work out, feed the filly good and make sure she has proper worming and lots of vitamin and minerals and she will grow properly, she will probably mature between 15 hands and 15.1hh. I have had colts out of 15 hand mares by 15.2 sires that reached 15.3, but have had some out of 15 hand mares and 16 hand sires that were 14.3 when grown, you never know where the height gene will come from.