For an exact fit, your dog should be standing square. The length is from the base of the collar to base of the tail, and equals the size of your dog coat. The Girth should be measured around the body from one inch behind the front legs. The neck should be measured around the neck on the base of the collar. Those measurements provides a general guideline for your dog. Most of Sirius Couture™'s designs are adjustable for the Girth and Neck to provide perfect fit.