When you say CKC.. do you mean continental or canadian? Canadian is reputable, but continental is a bogus registry. It means nothing. Lot's of backyard breeders and puppy mills use it (continental) to look legitimate to potential puppy buyers, but it's not.
Regardless, I wouldn't worry about size. Has she been seen by your vet yet? Is she getting her puppy shots? If the vet says she's healthy, I wouldn't worry about her keeping up with other dogs. She's fine.