Good Question! The truth is there is no such thing. There is no registry that recognizes a classification as teacup Yorkies, at least not to my knowledge. It is just a term breeders use to describe their smaller Yorkies and even at that every breeder has their own idea of what a teacup weighs. The breed standard for a Yorkshire Terrier is 4 to 7 lbs. so I have broken it down as follows:
Teacup - under 4 lbs.
Tiny - 4 lb. to 5
Average - 5.1 lbs. to 7 lb.
Large - 7.1 lbs. and up
Also just because a dog is small does not mean it is unhealthy as some breeders would like you to think. It all depends on how they are being bred. My small puppies come from small parents and are not a runt from bigger parents. I have very healthy pups and that is why I give a health guarantee.