You are right about the large breeds required to carry the knights in armor. They did not look so much like the modern clunky Clydesdale or Belgian-type draft horse, but they were large strong riding horses of some magnificence!
Two of the ones still bred true to that style today are the Friesian (commonly seen in medieval costume in demonstrations and shows):
And of course, carrying knights to battle was the original purpose of the Lipizaner Of Spanish Riding School fame. They are bigger than they look... Large in size but still with very fine features and grace:
