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.

You live "on" a hump, but "in" a bowl. Once something is big enough that it becomes larger or more irregular (no longer expressible as a single characteristic) than a bowl, you change from "in" to "on."

Therefore, "in" Africa or other continent. "In" Ireland. When you express the concept of a country as an island, you emphasize it standing out of the water and it becomes a hump, and you change to "on."

This can apply in the plural, so The Falklands can be either multiple bumps, or one political entity.

I believe this generalizes all the other comments made, even the one about Earth - a bump in space.

Listen to the sound of "on the British Isles" and "in the British Isles" and you will be able to discern the isles rising out of the sea in the first case, and being an indistinguishable entity in the second.