I voted 9lbs+.
Mine were all 9+ and like a pp said... the newborn stuff was too tiny for them! Plus I had to deal with the "big baby" protocols after each birth (turning down blood work, signing waivers, etc).
On the "official" end of the spectrum, ACOG guidelines for fetal macrosomia put the cutoff at 9lbs 4oz (it used to be 8lbs 13oz). So "officially" in a medical setting, a big baby is anything above 9lbs 4oz at birth. Though even the ACOG guidelines don't suggest a c/s for macrosomia unless the babe is 11lbs+. There's a nice summary of the ACOG guidelines here
in case anyone wants to share them with their care provider... I mean, if even ACOG doesn't support surgery unless a babe is in the 11lb range, where are all these care providers getting their pro-c/s info?
eta- just like pp said!