The CCA website discusses this topic for a full page.   Let me summarize:  There is no such thing.

     When people call and tell me that they want a "tea cup", I ask them if they know the technical name that Chihuahua breeders have for "tea cups"?  They say no.  I tell them: "We call them runts."  (Okay, but I smile when I say it.)

     And they are darling.   In a breed where our adults are the size of the puppies in most other breeds, these are the extra tinies, the especially miniature.  BUT ... THAT DOESN'T MAKE THEM THE MOST DESIRABLE PETS.

     And although Heaven knows I don't breed for tinies, occasionally one pops up.  And I don't get more money for them - I usually ask less.  The last one that left here was 6 months old; I didn't feel it was safe for her to leave any sooner.  And - I sold her for about 10% of the cost of her vet bills.  And I didn't give the new owner a health guarantee.  (A first, for me.)   Not to mention that I put the poor woman through the Inquisition.   Even worse than I usually do.   (She was a fine, knowledgeable, former breeder, BTW; she passed with flying colors. ;-)

     Sometimes I ask people who call with these questions "How big do you think a tea cup is?"  I get answers all over the spectrum, including "Four pounds."  Four pounds is a decent size for a show dog or a brood bitch.  

      So if someone is advertising "Tea Cup Chihuahuas"... RUN. 

      Here is just one paragraph from the CCA website on "Tea Cups":

“Unfortunately, the additional adjectives used to describe the size differences and physical appearances are many and have been misused for so long they now seem legitimate. Teacup, Pocket Size, Tiny Toy, Miniature or Standard - are just a few of the many tags and labels that have been attached to this breed over the years. The Chihuahua Club of America is concerned that these terms may be used to entice prospective buyers into thinking that puppies described in this way are of greater monetary value. They are not and the use of these terms is incorrect and misleading.” ....   

“The Chihuahua Club of America does not endorse nor condone the use of any of these terms and would caution the perspective puppy buyer not to be misled by them.”