Just out of curiosity, I have a rising 2 year old tinker horse. He is currently standing at 14'2 and 14'3/15hh on his bum. Any ideas on how big he is going to get?
I'd guess that he'll end up somewhere between 15.2hh and 16hh. You can usually add about a hand to their two year old height to get a rough idea of their mature size. The string test will likely give you a more accurate estimate though.