The most critical measurement is length measure from the centre of the horse's chest to the back of the rump (directly down from the dock of the tail).

Second, measure from wither to dock.

And for the neck, measure from the front of the wither (where you want the top of the rug to start) down the shoulder to a point level with the top of chest (where you want the front of the rug to come up to).  Ideally, you should also measure the diagonal between these two lines, to indicate how open the neckline should be.