Hi Jane
I have a smaller yorkie. Ziggy is 3lbs and 14 months old. When I get my second yorkie I would like her to be around the same size as Ziggy so they play well together. My boy has been very healthy.. BUT he was bred by a BYB and the second puppy I got from her died of liver shunt (he was bigger than ziggy though)
In my own opinion, I believe the health of a dog is more likely determined by the quality of the breeder. A breeder who breeds for tiny ones, will probably have more sick dogs than one who just so happens to have a smaller puppy.
I am already on a waiting list, and I chose the breeder because she is very reputable.
So to answer your question, I think that if you know that this is a good breeder who has tested her animals and has a good health record, i say go for it. BUT just note that the small one's do require a lot more attention, I am home all day with Ziggy and will be home too when my next one comes. It is also a good thing to let the puppy stay a bit longer with the breeder to get stronger and also do AS MUCH research as you can on taking care of the smaller ones..
Good luck in your decision, but I think whatever you decide you would have thought it out and be prepared
