I totally understand you being nervous, but I guess the best thing you can do now is wait and see. I'm sure everything will turn out ok though. One good thing from what I read in your post is that the breeder wouldn't give you the pup until it was 3 months. A lot of breeders give up the babies at 2 months, but after being on this discussion board I've read and learned that 3 months is really better.
It is hard to predict their exact size, but from what you said (a little over one pound at 3 months), it seems like your baby will be quite small. My little guy is Hercules and he's almost five months. When I got him at 10 weeks he was 2 pounds and at 4 1/2 months he was 3.8 pounds. Just to give you an idea of another yorkie.
Well good luck and make sure you tell us how everything goes when your little one arrives. Do you have a name picked out for her yet?? Make sure you post pictures too!!
Brandi & Hercules