Yorkies are not really meant to be really tiny. Many times those that are, have health problems that can be very costly and there is no guarantee that the pup will have any sort of life longevity or quality of life. On occasion, one can be small and be healthy but that would be hard to find and be sure the one you get is okay.
You can visit the website for the Yorkshire Terrier Club of America at
www.ytca.org and click onto their Frequently Asked Questions page. There is info there on 'teacup'.
A Yorkie that is about 5 lbs to about 7 lbs full size is still a very small dog and much more likely to be healthy.