Originally Posted by agilityk9trainer
Actually, you see more undersized agility shelties than oversized ones. As has already been posted, you can get an undersized sheltie pup and an oversized sheltie pup in the same litter. It's the reason being undersized or oversized will lead to dismisal in the show ring. They are trying to get the breed more standardized in size.
However, many agility enthusiasts prefer the smaller sheltie. In AKC, the 12" jump height allows for dogs ranging in height from 10" to 14". Many sheltie agility folks want to jump in the 12: height, and get a pup that they know will be small. The 16" inch jump height allows for dogs ranging in height from 14" to 18".
You asked how big Aslan, my MACH sheltie, is. He's undersized at 12.38". However, my color-headed white, Jericho, is oversized at 16.5". Both had parents that were well within the standard for size.
I don't know any breeders who intentionally breed for large agility shelties, although I could see a demand for small agility shelties. They are very popular.