I go by height. I consider Colbi to be in between large and medium, she's taller than a medium and shorter than a large dog. I'd say Cocker Spaniels/Border Collies (that's all that comes to mind right now, I'm kind of out of it today. Playing my saxophone too much. ) are medium sized, and then Labs/Golden Retrievers, Huskies would be large. Then there's Newfoundlands, Saint Bernards, Great Danes who are extra large.