Without a point of reference, height of your great grandfather or what type of horse it is to estimate it's height, any quess as to the mans height is pure conjecture.
However tall he was, he was a big man. If he were born 70 years later, I'm sure he would be recruited to play some B-Ball and possibly be a millionaire if he were good enough.
EDIT: It looks like a Draft horse, which range approximately between 16-19 hands (one hand eaquals 4 inches) and is measured at the whithers. (point where the neck meets the shoulders). That is a pretty big spread, so to use it as a point of reference even knowing the breed is probably not much use.