I agree with Hendryk. I see no reason while a KMT China, without atrocities like the Great Leap Forwards & probably a less rigid one-child-policy would end up with less population than in OTL.
However, I will reduce my earlier estimates of 2-2.5 billion Chinese in 2009 to 1.4 to 1.6 billion.
That's reasonable. You have to consider that KMT China is probably going to pull an Asian tiger much earlier than PRC China so it will experience the demographic transition earlier, maybe as soon as Japan.