Yep, that was the Winn Foundation link I posted MM.
Spaying doesn't affect their growth, all cats are different. I have 4 females in the house, ranging from 2.5kg up to 3.7kg. Females do tend to be smaller than males though. Spaying at 6 months (well, I think 4 personally) can have health benefits when it comes to mammary cancer - if you spay them before their first season (which can happen as young as 4 months), then they are 200 times less likely to develop it.