To ensure proper fitting equine apparel (sheets, stable blankets, etc) please use a string or soft measuring tape, and measure from the middle of the horse’s chest, around his shoulder, across his barrel, around his hip, and to the edge of his tail/point of buttock.   Make sure the measuring tape stays horizontal to the ground and goes around the widest part of the horse.  The number on the tape is the blanket size, round up to an even number.  This measuring technique is often easier with two people.  Blankets and sheets with buckle fronts offer some adjustability in sizing.