How to measure a horse for a collar:
The old way is to take 2 straight edges - place 1 on top of neck and 1 on bottom of neck, parallel to the shoulder blade, measuring between the 2 straight edges in a straight line. This should give you a close measurement for a collar. 
The new way: (I was told about this from a harness maker in England.)  Measure the circumference (widest spot on the horse's front leg, usually under the armpit).  This has been giving us the collar size, within an inch or so.


How to fit a collar to a horse:
If a collar is not properly fitted to the horse, it is sure to make him sore.  Therefore, it is necessary to use the greatest care in selecting the proper shape and size of the collar.  The illustration below shows you how a collar should be fitted.  If care is taken in fitting the collar and adjusting the hames so the hame tug comes over center of the draft, you have eliminated all the trouble of sore necks or shoulders, and the horse will work with ease and comfort. 
  When fitting a horse with a collar there should not be any more space between the horse's neck and throat of the collar than will allow your fingers, when laid flat on the inside, to pass through freely.