How to measure your Draft for a Dressage Girth
First, set your dressage saddle on your mount's back minus the saddle pad. Ensure the saddle is in the correct spot by fitting the deepest part of the seat into the deepest part of your horse's back.
Second, position your measuring tape on the third hole from the bottom of the left billet and secure it in place with a piece of masking tape.
Third, pull the measuring tape under your horse's ribs up to the third hole from the bottom of the right billet, fitting the tape snugly as you would a girth. This measurement gives you the size of girth you need to purchase.
Measure your girth so that when you buckle it, you can fit two fingers between the girth and your horse's ribs. A too-long girth can cause your saddle to slip when you mount and be unsafe for riding. A too-short girth binds your Draft's ribs and lungs and can cause sores where it rubs the skin.
Best guess, subtract 20 inches from your normal English size. Not an exact science however, since different brands of saddles have longer/shorter billets.