Medium Dog Harness. This is a simple A Type Harness - your dog simply steps in, you fasten the clip, attach your lead and you’re ready to go. They are much more comfortable to your dog than just a collar pulling on its neck. The size is for the girth of the dog (around it’s chest and back): Medium - 18-23" (46-59cm) The Harness is made from a 25mm polypropylene webbing for extra strength and durability, polypropylene is rot proof, U.V resistant and doesn't retain water. They are also resistant to most common chemicals.