So long as the horse is agile and quick, then it doesn't matter what height he is. The biggest problem with that is, larger horses are, in general, slower than small horses. Larger/taller horses have that much more bulk and leg that they have to coordinate in order to get those quick turns.
That being said, my next ranch horse is standing just over 16.1hh right now...and weighing about 1400. He'll work great for what I do because I do more slow sorting and roping than I do cutting type work. It would be quite a pain in the butt trying to turn him into the next High Brow Cat