umm it really depends on how much your pup weight when you get her not by its age. I would take your pup with you to petco or petsmart and try some on him/her. My bunny would in a xs at 10 week but now is in to small. My bella was a XXS and now is a XS...
oh and the hair growing long thing really depends on your yorkie. My bella had longish hair scent 6 weeks, some still dont have long hair at 5 months. So it depends on your yorkie.
There is a weight chart here
http://www.yorkietalk.com/forums/showthread.php?t=16045... might want to check it out
Good luck getting your furbaby! cant wait to see pix
