Some breeders use names such as: Toy Yorkies, Tinies, and Teacup Yorkies. These names have been invented to market the different weights of Yorkies.
If you are wanting a female Yorkshire Terrier to breed, you should get a female that is a few pounds larger than the male.
When looking for a pet for your child, a larger Yorkie of 6 to 8 lbs. would be be less likely to become hurt during play.
My yorkie parents range from 4-7 lbs. I use parent weight as a predictor for adult weight of the puppy. NO one can promise you that your puppy will weigh a certain amount when it is grown.