Size wasn't an issue with my girls...I have teapots
I know vets have different feelings and rules regarding size so talk with your vet. There is a sticky post in the Health and Diet forum called "Anesthesia Protocol" with information and questions to ask your vet.
Many menbers here have had their small girls spayed with no problems so maybe they will step in and be able to help. I will say though that my little male, Peanut, was neutered when he was only about 2 1/2 pounds but I don't know how different it is for males than females. He did have a more invloved surgery since he was chriptorchid so it wasn't a routine neuter.