First: Stand your horse on a hard leveled surface making sure that he or she is nice and square.
Second: Using a soft measuring tape or a long piece of string, place one end in the center of your horse's chest. Measure from the center of the chest, along the left side, to the middle of the tail. Be sure to cross over the point of shoulder (widest part of the shoulder), and that you keep your tape pulled tight. If you used a piece of string, once you got the length, simply compare it to tape measure to get your horse's size.
Measuring is usually easiest with the help of a second person, however if you have to do it alone, draw the tape along your horse's left side as far as you can, once you have reached as far as you can, mark the spot with your right thumb AND take notice of the measurement. Then restart measuring from that point to the furthest point of the rump, then simply add the two measurements together to get your horse's size.
Most stock blankets come in even sizes, so if your horse measures an odd number, round up to the closest even size. For example, if you horse measures 73", use a size 74 blanket.