You are doing the right thing!
The veterinarian knows what he or she is doing, even if something does go wrong. I've never heard of anyone losing a dog during a spay. Luna is just under 5 pounds, and she seems to have reactions to everything. While coming out of surgery she had a reaction, but the vet and the techs handled it and she was fine. If you are super worried, ask your vet for an anesthesia test. It is the anesthesia that small dogs might have trouble with, like Luna. They can give her a small dose to see if there is a reaction or not. It costs extra, I think $40 or $50, but is worth it if only to put your mind at ease.
Your baby will be fine.
Luna+Ichi