As luck would have it, there's already a CK2 China mod (for the three kingdoms period, no less!)the ModDB pagethe Paradox Plaza page
(requires a login)
IIRC, the map they use is on the paradoxplaza page, a few pages in. And in either case, the general rule is "the denser an area's population is, the denser the counties are in that area". It's the same reason why, as of 189, the physically smallest commanderies -- and the five physically smallest zhous -- are all in the chinese plain!
Unless I specifically say otherwise, assume I am talking about historical Three Kingdoms, and not the novel.