Find another Vet. If you can, get in touch with toy dog breeders in your area especially a yorkie breeder. You could try the YTCA breeders list and find out where they go for Vet care. YTCA is at
Yorkshire Terrier Club of America Sometimes it takes a Vet familiar with the breed. I have had 3 1/2 lbs females spayed before and no problem. But if yours happens to have some other health issue, which you would want to find out if she does, then anasthesia may be a problem.
Correct use and type of anasthesia also can make a difference.