Sheltie Question???

I am getting a 12 wk old sheltie puppy Sunday and I can't wait!! Anyways both of his parents are 15 inches tall....is there a chance that he will get bigger then that? At what age are they full grown and when do they start getting their pretty fluffy coat? The lady I'm buying him from said that she's never had a oversized sheltie and I've never owned one. Thanks