I picked Lily out at three weeks old on a picture. At around 6 weeks I went to visit her and her sister. The sister was still available and DH really liked the looks of the sister. Lily was smaller than the sister. When I got to the breeders home, she brought out both girls. Lilys sister was whining. Lily came right to me. I picked her up,, she crawled up my chest, and started licking my face. I knew at that moment she was the pup for me. I think going to visit was the best thing I could have done. It was at that moment I knew I had made the right choice.
When I first started looking, the breeder was really encouraging me to take the smaller one because she knew I had other Yorkies. It really would not have mattered which one I had got. I would have loved either one. However, from our meeting, Lily fit what I was looking for.
I would wait a few weeks and go pick him out once you can see his personality.
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Also I was told the runt sometimes takes a growing spurt and can be bigger than the others. I think it's a guessing game on the actual size lol.