You can measure her front leg and tell how tall she will get. If she has not received much protein in her diet, she could grow taller than that measurement but she will grow AT LEAST that tall.
Take a piece of cord or thin rope -- something that isn't too 'stretchy'.
Place One end on the round knobby part of her elbow (right in front of her girth).
Extend the cord down her front leg to the very center of her fetlock joint. Hold that spot carefully on the cord.
Then reverse the cord -- still holding the one end on her elbow.
The end that you are holding that was at the center of her fetlock joint should extend a good bit above her present withers.
She will grow AT LEAST that much.
Two year olds and older horses will only grow more than that if they did not receive adequate nutrition as weanlings and yearlings.
If you doubt this method or have a question of exactly where you measure from, go out and measure all of the mature horses you can find. About 99% of them will be exactly that height when you hold the cord at the center of both the elbow and the fetlock joints.
It works for leggy horses and short legged 'dumpy ones alike. It does NOT work for mules. I did a three year project on this and measured over 600 horses from age 1 1/2 to 6. I went back and remeasured the ones I could find. It was over 99% accurate for the ones still growing. The ones that showed they would not grow more, didn't. They were mostly well fed 2 and 3 year olds.
I did not measure any 'minis' because they were not around back when I did this. I do not know how it works on them. I measured mostly horses that were going to mature in the 14 hand to 17 hand range. I only measured a few drafts, but it worked pretty well for them, too. It worked very well for the QHs, TBs, Arabians and other saddle horse breeds and types.
Measure this filly and let us know how much she is supposed to grow.