Here's my 2 bits...I am always doing full scale earth-size maps at 25,000 X 12,500 pixels. My PC really can't handle that amount of info so what I do is do up my initial height fields then chop it up into smaller bits of 5,000 X 2,500. These smaller chunks can then be worked and posted up for getting feedback. One vital thing of importance is to write down your steps so that you don't forget the process from one map to the next 'cuz sometimes it takes a few weeks before you can get back to mapping.

Cheers.