horse measuring “a head” is not a specific height, it is the height of that individual’s head). you can “eye it” (use your eye to estimate how many heads tall) or you can use your pencil and thumb like a ruler to figure out how many heads tall and wide a person is.
look at your person again (if you are in the room with someone else it will be better if you actually walk over to them and a piece of paper or pencil to measure their head then how many heads tall they are.) also measure how many heads wide they are at different points. how many heads wide 