I read your post with interest because I have a five month old female who only weighs about 3 lbs. I am also concerned about having her spayed because of her size.
The size of a dog doesn't necessarily have anything to do with when they go into heat.
I plan to wait until Taz is six months old and see how big she is then. Then I am going to make an appointment with the vet for a checkup for her and talk to them in person regarding having her spayed. I'll ask if there are any risks with a smaller dog and what they do to minimize those risks. I may also decide to wait to see if she grows a little more. I'm not worried about her going into heat first. She uses peepads in the house and my only other dog is a neutered male and if we have to endure one heat before I have her spayed that's okay.
That said, there have been many others on this forum that have had tiny females spayed and they didn't have any problems. That does reassure me and I hope it makes you feel better too!