I would just break it up by continents, each continent a separate file and they when they are all done bring them as a flattened image back into the main file. the ocean would be the only thing I would keep in this file, making it easier to nify the whole image and texture in one image.
Also check out whole world maps in real life. To region maps I would just crop and resize and use this as a base for the region map. Don't forget you'll have rationalized some of the shapes for scale purposes so any regional map will reveal more details that wouldn't be seen on the world map.
Just look at the details you see when you start zooming in on Google maps. Focus in on how you see the details on places like Norway emerge as you zoom in closer and closer. Or scotland...and see when you start seein all the smaller isands that surround it actually visually discern them on the map...
So in short... it's not the same map. Just crop and rezise and have fun making up the extra details