The small- medium is 0.75" wide and can expand from 14" to 25" in circumference, so it will fit a medium sized dog as well as a small dog.
The medium-Large harness can expand up to 33 inches and shrink down to 20inches, so it will fit a medium sized dog as well as a large dog. They are printed on 100% nylon, and the quick release buckle is plastic. The image used on the harness is taken from one of my favorite tapestries. This design will only be around for a limited amount of time.