...for maps larger than 100x100.

press "m"...

In-game, I assume? And then what? Open a graphics program and save-as?

No, go in the editor, load your map, press M then go to View, then go on Zoom and click 25%. It won't actually get you a picture of the whole map, though. Don't know if there is another way.

So I'm stuck with a 70x70 or so map if I want to get the entire world in one shot...

Take shots screen by screen, covering the whole map piece by piece (you can even do that in-game since it will look better). Just paste each screenshot in Photoshop (or whatever you use) after capture. It is tedious and jigsaw like (you have to make each separate SS match with the others), but it works. Example of what it can look like attached (blatant advertisement for RFRE, sorry I can't help it ;)), although resolution and size have been bring down to fit with imageshack requirements:

http://img453.imageshack.us/img453/1500/mapsmall6mg.jpg

I'll try that,thanks!