It does not matter how well a horse collar is mad, if it is not properly fitted to the horse, it is sure to cause scalding.  Therefore it is necessary to use great care in selection the proper shape and size of collar.

To measure a horse for a collar take two straight objects, such as carpenters squares, to form a C. Set along the neck of the horse where the collar would sit with the top of the C  at the withers and the bottom at the point of the shoulder. (illustration 1) Then measure the inside distance from top to bottom. This is the size of collar you will need. 

A collar is measured on the inside from the top to the bottom on the rim side.  (illustration2)

When fitting a horse with a collar there should not be any more space between the horse's neck and the bottom of the collar, than will allow your four fingers, when laid flat on the inside of the collar, to pass freely.