Im pretty sure its like measure from the fetlock to the knee than times it by something and it will give you a "guess" as to how large your foal will be, but im not sure at all. It was something a blacksmith told me a while ago.
A good bet is to see any other progeny the dam has thrown. Although we had two full sisters, one was a solid 16hh and built like a tank and the other was around 15hh or shorter and built like a torpedo so its a hit and miss.
Simplest way i think is to take the dam and the sires heights and average it out. Nothing is guarenteed but it gives you a little helping hand lol.
A Dog looks up to a man,
A cat looks down on a man,
But a patient horse looks a man in the eye and sees him as an equal