A saddle with a suitable gullet size is just as critical as a seat size for the rider. Gullets come in a range of sizes to accommodate the width of a horse's back. Peering down from above on your horse's back will give you a good idea of the size you'll need. An average-sized horse typically requires a medium-sized gullet. You should calculate your horse's withers if you're not sure what size gullet he or she needs.