I always make my maps as big as the largest I can conceive of printing it * 300 dpi. For instance, my current map is sized to 22" x 22" - so it's 6600x6600. That way, if I want it small, I can shrink it, and if I want it printed huge, I have it. Because you're right, enlarging is not really an option. *Edit: I don't usually go to 2x final resolution, because I usually work at 300% zoom (pixel for pixel on a ~100px/inch screen) or higher (200% zoom on that).