I have four tiny yorkies... two of them I purchased when they were a little older... My Jasmine at 7 months old... and my Butch at 4 months... sometimes you can tell by the feet/paws whether they will be little.
It's hard to tell from a pup what size they will be... and most breeders cannot guarantee the size.
Now
this is small... see my Bitsy's pup...he will be 4 weeks old on Wednesday and weighs 4.4 ounces!!

He looks like a hamster next to the other pups....
Best of luck with your pup.