How small does a land-mass have to be before you live "on" it, rather than "in" it?

In my experience, this is often based on whether one is speaking of geographical versus political locations, and also the context of what you're saying. You would be "on" an island or continent or planet, but you'd be "in" a country or city or region.

So:

• One would live in England, China, Canada, New York City, North America. (Political locales.)

• One might be located on the British isles, Manhattan Island, on the continent of North America, the planet Mars. (Geographical locations.)

It's kind of fuzzy, though, because I've heard of people living in Africa or Antarctica. Can anyone think of exceptions to this—this is English, of course there will be some—or help clarify further?

Edit: Wow, that's quite the discussion going on in the comments!

I'm convinced that while this answer is an extreme generalization, it does seems to serve as a good starting point, if nothing else. Like everything else in English, there are no absolutes.

• There's some consensus that multiple islands (Hawaii, Japan) forces a political interpretation, and you'd live in Japan but on the island of Honshu (although you'd live in Honshu).
• There's also some disagreement about whether you live on or in a continent.

I live in Manhattan. I also live on Long Island. Bear in mind that Manhattan is much smaller than Long Island.

Would you live on the Napa River Valley or in it? I think it may be more related to the containment of the item/object in question. For example, I can stand on the ground, atop a rock, but in a hole, river, or valley. Similarly, fish live in the ocean, but ships float on it.

In this case it might be more appropriate to think of in as within. I'm not going in the door, I'm standing within the walls of the house. The shortened version of that being, "I'm standing in the house."

The same could apply to continents and why the accepted answer notes that some may make reference to "living in Africa." This is because they are within the borders of the continent of Africa.

It also depends on the physical geography. You live in the Dales (valleys) but on the Moors.