The smallest female I've ever had spayed was my 13lb Schipperke. Oliver, my shih tzu, was 5 months and 7lbs on the day he was neutered and he was just fine. (Of course, I was a wreck
)
I've had a 4 month old 3lb female cat spayed also. She did wonderfully, too.
It's harder on us than it is on them. That said, Belle will be 6 months old on Sunday and can't be more than 4lb 8oz and I'm afraid to have her spayed, too.