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. I have very healthy pups and that is why I give a health guarantee.