I know many people who have had their small girls spayed without problem. Make sure you have the pre-op bloodwork done in advance and discuss the anesthesia protocol with your vet to make sure the best is used.
Here is a thread about the anesthesia protocol:
http://www.yorkietalk.com/forums/yor...-protocol.html
It's natural to worry. I was a mess when my big boy was neutered, and I wimped out with my second boy and had the breeder take care of everything before we brought him home at 8 months.