Many trainers prefer harnesses with full collars, or hames collars, over breast collar harnesses. Collars allow the horse to support his load with his whole body while keeping his shoulders free. Unfortunately, full collars are difficult to fit, and the fit must be precise to avoid injury to the horse. If the collar is too wide, the pressure will be uneven, resulting in a rocking motion and causing sores on the shoulder. Too narrow can constrict circulation, causing muscle cramps, pain and stress. Too short a collar puts pressure on the horse's windpipe, while too long will cause sores on the point of his shoulder and the collar to rise up. Proper measurements only require a few steps.
Lay the carpenter's squares back-to-back so they form a "C."
Place the squares so that the "C" brackets the horse's neck with the top arm in front of the horse's withers and the bottom arm two inches in front of his shoulder. Be certain the bottom arm clears the base of the horse's throat by at least two inches, and the back of the "C" lies along the angle of the horse's shoulder. Measure the distance between the two arms and record it on the paper. This is the approximate length of the collar you need.
Tape the carpenter's squares together to hold them at that measurement.
Replace the combined square at the same point on the horse's neck as in step 2. Place your fingers between the side of the combined square and the horse's neck so that the square is a finger width from the widest point of his neck. Find the midpoint on the horse's neck on the upper and lower arms of the square. It should be the same above, at his withers, and below, at his throat. Record that distance and double it. This is the approximate width of the collar you need.
Once you have located a collar with the correct measurements, place it on your horse. If the length is correct, you should be able to just slide your hand into the two inches between the bottom of the collar and the horse's throat. If the width is correct, the upper sides of the collar should fit just tight enough to allow no more than a finger width between collar and horse all the way to the cap.
Try several different used collars until you find one that fits correctly all the way around the horse's neck. You can then either have that used collar re-padded and reconditioned, or use it as a model for a new one.