Small: 63" to 69"
Medium: 72" to 78"
Large: 81" to 87"
How to fit your blanket:
Using a soft measuring tape, measure from the center point of the chest, across the point of shoulder, and end at the center of the tail. This measurement should be taken in inches. When you have that measurement, remove 4 inches and that is your horse's blanket size.
Every horse is a different shape, so bear in mind that they may go up or down a size according to the fit and cut of the blanket.