Hi Krebs :)
Size of imported images can be whatever you want - the app will scale them down to it's maximum size automatically.
If you want to make them up to the max size yourself, then it's 2048x2048 @72dpi.
As long as neither direction is no more than 2048 then it's fine. They don't have to be square.
I have been making them as 24bit PNG files, and they end up around 4 to 6mb. This is fine tested on iPad1 and iPHone 3GS and up. Confident about Android too (I mean, at least similar kind of spec to 3GS) but as there are so many devices we have not tested on EVERYTHING!
In terms of getting 100's of miles into that size, I guess it depends what level of terrain detail you need. The ARMA map is pretty big and it's OK at zooming. Obviously you don't see individual buildings but it's enough to organise the troops! For flight, I guess you have either individual targets, or much larger 'zones' of importance. You can use the circle tool to create zones and you could use the target icon which has a precise centre-point for marking building targets etc.
Anyway I would be interested in your thoughts!