Ironically, when we were looking for an adoptable Sheltie a few weeks ago I was set on a small to medium dog, due to back trouble. But Cosmo was described as well trained on a leash, and gentle, which he is. He weighs in at 35 pounds according to the shelter, and although some of that is probably fat he's taller than any of our other Shelties. We love him, and like you I don't think we would have had a chance to adopt him if he hadn't been oversized for showing (also, a color-headed white.)
When we were looking, we found one ad for a 60-pound Sheltie and a breeder who was trying to place a 40-pound girl. The big ones can be really big, and my understanding is that one litter can produce all shapes and sizes.