Well.... Hm, there were mountains in the North of No. 6, right? That helps narrowing things down a bit. Forests and plains can be manmade, mountains, not so much.
I think we can rule out all of South America and most of Africa, and definitely Australia. Countries about the height of Indonesia as well, I suppose... And everything too far up North, at the height of Iceland, considering the amount of day hours and the general cold/snow. Oh, and an area that gets tropical storms, I suppose. That might rule out Europe?
Oh, hang on, from all the way back in volume 1:
a tropical low pressure-system, or hurricane, that had developed a week ago off the South-Western area of the North Pacific Ocean, made its way north, gathering power, until it hit us directly in the city of No. 6.
Off the South-Western area of the North Pacific Ocean - that would indicate East Asia above the equator, and No. 6 lies above the point where the hurricane (or typhoon as it should be called) formed. Here's a
map of typhoons between 1980 and 2005.
So if it's indeed a tropical storm and thus a typhoon, then this is the area we should be looking at. Japan looks likely, as does China - maybe also Korea?
(Disclaimer that my topographical and geographical 'knowledge' dates back to junior high and is WOEFUL, so if someone is more informed about this than I am, please feel absolutely free to correct me.)