I ordered size Large because my dog had a 26 1/2 inch chest. It was way too big, & I had it as small as it would go. Good thing I have a slightly bigger dog that it just fit on the smallest adjustment. I decided to order the medium, and it fit perfectly, and I didn't even have to make it as big as it goes. The only thing that is disappointing is the medium is only 3/4 inch wide. I was really hoping to have a 1 inch wide harness. You also have to be very careful not to twist it or you may never get it back to normal. I've had other brands of step-ins that seemed a little better quality. Overall it's a good harness and I love the pattern. I would order this brand again.