Peblo's is certainly the most versatile of the bunch.
Anyway, Lukas, the way I've got it set up, no actual frame-scrolling occurs in the game, so the frame-size limit doesn't apply. Instead, everything is loaded on a page-based system (each full screen is a page, and four of them are read at one time), and, since I'm using one byte to store each of the level's dimensions, I can get up to 256 pages in each direction (although I haven't actually made any levels that big, so for all I know it might start acting strange after 128.)
So 256 by 256 pages, pixel-wise, is... *gets out calculator* 81920 by 61440 pixels.
'Course, if you really needed the room, you could add another byte to each dimension, getting... 65536 pages and 20971520 pixels across.
So it'd definitely be possible to get levels that large. It's just a matter of pulling it off.
As you can pretty much generally make up the filetype it varies from game to game. If your game was called Pac-Man I'd assume the extension would be .pml for 'pac-man level'... It doesn't really matter but that's what people generally like to do