To measure the hackamore, first find the top of your horses nasal cavity with your fingers. You will be able to follow it up and feel where it stops. Then place a piece of string or measuring tape one finger with above the spot where the nasal cavity ends. Take the sting/measuring tape from the center of the nose down to the groove under the chin. Go under to the center. This will give you half the circumference of the face. Double that number to give you the full circumference in the place where the hackamore will lay. With that measurement you can choose the right size for your horse.
If you already have a hackamore which fits your horse, please measure it as illustrated in the second picture. Hold the front part 4" apart (horizontal arrow) and then measure from the inside of the nose knot to the inside of the heel knot (vertical arrow) to get the correct length.