Welcome to YT. As for size it is really hard to tell. People have chosen the smallest and it has grown to be the largest. My Gus was already two pounds at eight weeks - I was expecting him to reach six pounds maybe a little more but he stopped at five.
The only way to ever be completely certain would be to get an older puppy. Sorry I couldn't be more help. Good luck finding your puppy - you can't go wrong no matter what size a Yorkie turns out to be.
