Well, Sprite's about 2 lbs 5 oz (dwarf hotot) and she did great with her spay, as did her 3.5 lb sister. Mocha was just under 2 lbs (4.5 months old Polish) and while she did a picky princess routine she did well too.
I think age/maturity is more important. A rabbit that is too young won't have mature organs and will be harder to do surgery on. But those little dwarfs mature so fast. Mocha was only 4.5 months old but the vet said her reproductive organs were fully matured and perfect.