As "unified China" I mean lands where most people live, so Tibet, Gobi desert, Manchuria and Taiwan can be excluded.
I have the recollection that in the early 5th Century BCE, the Achaemenid Empire of Persia contained 44% of Earth's roughly 112 million people, which, when one takes into account other humans in Africa, the Americas, Western Europe and what is now Russia, probably means it outnumbered China.