To me, a medium sized dog is 25 to 50 pounds. Small would be 25 and under, large between 60 and 100 and giant over 100.

So to me, kelpies, ACD's, Border collies, and aussies are medium. German shepherds, labs, tervurens, malamutes are large. Chihuahuas, mini and toy poodles, shih tzus are small and Irish wolfhounds, great danes, bernards, mastiffs are giant.

Of course it depends on the individual dog. Normally I'd say goldens are large breed, but I've seen some small goldens that I'd classify as medium, same with german shepherds.

And if you go for height, if the dog stands at about 3/4 or less up my calves, it's small. Medium is just below my knee, at my knee or just above my knee (top of my knee measures 22 inches). And large would be well above my knee more like 1/3 of my theigh. And then something like a borzoi....well a borzoi's withers would be just below my hips and their head roughly at my chest.