Use a flexible measuring tape. Start at a point directly beneath the end of the cheek bone. Measure from this point on one side of the nose to the same point on the other side of the face.
Starting at the same point (directly beneath the end of the cheekbone) lay the measuring tape flat on the horse's face and measure up the face and continue over the poll (behind the ears) to the same point on the other side of the face.
Match your measurements to the closest size shown on the