I think it depends on what type of breed you personally would like more. I've always loved yorkies. I think once you see the dogs, your heart will just know. I knew Chloe was going to be mine from the very first second that I saw her. She was predicted to be 3 pounds full grown and that's how it ended up. She was about 1 pound at 8 weeks. Sometimes the predictions are inaccurate, though. Here is a growth chart that can be a rough guideline for you:
Yorkie Growth Chart | Yorkshire Terrier Growth Chart
I also don't think it is true that smaller dogs are more prone to health risks. Although genetics play a big role, how you take care of your dog and what you feed plays a role as well.
Good luck with your search.. I'm sure either breed will bring you a lot of happiness
