miss buffey was spayed right at 6 months and i think she was 2.8lb. she did great and had no problems with the procedure (she did have a reaction to the pain meds which required a visit to the ER vet on easter morning costing about $450 - doh). she healed very nicely and it was hard keeping her non-jumpy and calm during her healing process. i wish i HAD waited a bit longer to spay her but in the end, it was fine
my other little girl was 8 months old and a little over 3lbs (about 3lbs, 2oz) and she also did great. the breeder had this done before she came home to me per my request
the only reason i would recommend waiting a bit is to see if you baby will need any baby teeth pulled. if that is the case, it can be done during the spay. as far as her weight goes, as long as your vet is experienced and feels comfortable doing surgeries on smaller animals, you should be fine
hope that helps!