To find what size rug your horse needs, please take a measuring tape and place in the centre of your horses chest (where the top chest buckle would sit) past the shoulder, along the side towards the rump to the longest point, making sure to stop before you get to the tail.
Using the tape as a guide, check where you would expect the horse rug to finish and see how it relates to imperial sizing. For example, a 6'0 rug will measure 6'0, so considering the 3-inch intervals in sizing that horse rugs follow, and you can get an idea where a 5'9 or even a 6'3 will finish.
HOOD SIZES
4'6 - 4'9: Small Pony
5'0 - 5'3: Pony
5'6 - 6'0: Cob
6'3 - 6'9: Full