In line with the above, I'm not sure how China could hold all of the areas you mentioned at the same time unless you can come up with a significant butterfly before 500.
I didn't think there was a limit on the POD. But I was imagining some sort of semi-independent autonomous polities within a broader Chinese state. It certainly isn't too hard to keep Mongolia in China with various PODs for Russia.
That surely applies to North America and eastern South America, no?In other words, Russia is able to retain its size mostly because most of its territory consists of Siberia, which means that it is sparsely populated. In order to have a state which is of comparable size, you would need to find a large region that was barely populated to begin with, and almost no competition from other countries. The latter is probably why North and South America could not have hosted such a large country, not to mention geographic conditions.