My filly has just turned 3, she is currently standing approx 14.2hh, late maturing breed (Lipizzaner) her dad is 15hh and her mum a good 15.2hh, she isn't a first foal, so how big do you think she wil be
Do the string test!
Measure from the point of the elbow to the point of the fetlock and then reverse the string from elbow to withers and the difference is how much she has to grow. Not exact but very close.