Only downside: if your pup has medium or longer length hair, it may cover most of the collar which makes it tough to see. I bought a medium for my 50 lb. Aussie and it's extended at full length, but fits nice and loose. I bought a large for her Berner buddy and it fits him with room to spare. Only other notes: it's easy to use, but the D-ring knocks against the plastic buckle when she runs and makes an audible knocking sound. If you pup is noise sensitive, a piece of cloth tape wrapped around the apex of the D-ring may help.