It is important that the horse boot or guard perfectly fit the horse. To measure the horse’s leg, simply start by measuring the circumference of the cannon bone at the midpoint. Now proceed with following the size guides given on the manufacturer’s website. These online guides and charts give you easy measurements and sizes of different products. Remember your horse size will ultimately fall under any three categories; small, medium, and large. Here a small size boot is for a horse that is 14-15 hands tall; medium is for 15-16 hands tall and larger is for a horse with 16.1 or more.