The Canadian standard calls for 13-16 inches, so that is what breeders should be breeding for. It's not unusual to see a larger Sheltie though. It's believed that bigger Shelties are popping up because Collies were used when they were establishing this breed.. and I believe (but don't quote me) that this breed isn't very old, which could be why there are so many different looks and sizes.

And of course, it depends on the individual breeder. The line Kai comes from has dogs that all look alike. I can spot a Satelier dog in an instant. The majority of his dogs at his "kennel" weren't very small at all.. probably pushing the 16" limit.

It depends on what the breeder is breeding for too. Shelties can be great at flyball and pretty good jumpers for their height, so flyball enthusiasts may breed smaller Shelties for smaller height dogs.