The fit of your blanket is extremely important in order to ensure the comfort of your horse and to prevent rubbing and slippage of the blanket. As horses of the same height at the wither come in a variety of shapes and sizes, it is useful to take certain measurements of your horse.
Measure the distance from the center of the chest across the point of shoulder to the center of the tail. Take 4" off for Rambo Turnouts. Using this guideline will mean that your horse will probably require one size larger for his turnout than for his stable blanket. Horseware blankets are sized in 3" increments. This measurement refers to the distance from the center of the chest to the rear of the quarters where you expect the blanket to finish.