Teacup is just a word, slang, it isn't an official description.
Most of the ones in the show ring here are in the 5-7lb range, some of the males are a bit smaller, but breeders who show won't usually keep a smaller bitch as it's too dangerous to breed from her. Even if you were to mate to a tiny dog there is still a risk of a big puppy because of the larger breeds like the Clydesdale & Paisley from which Yorkies were developed. Compared to many British breeds, it is still a relatively new breed & not as stabalized as some of those that are hundreds of years old.
All you need is the breed standard, it contains all you need to know...
http://www.thekennelclub.org.uk/item/199