If you are just short of the withers, the horse is probably17hh or 17.1hh which is fairly large horse.
I figured that out with this method:
Each hand is 4 inches.
5' is 60 inches. Since you are 5' 6" that means you are 66inches tall.
66 inches divided by 4, which is how large one hand is, is 16.5, but since each hand is only four inches, and we got 16.5 that means, you have to go up to the next hand, meaning, you are, roughly 17hh yourself. If you are just short of your horse, then your horse is probably about 17hh or 17.1hh.