Take a look at my GigaUSA map...I used essentially the above process to get me started (it was too daunting to try to redraw the entire map).
Start with a large map. Use the MapEditor utility to save as a bitmap. Micosoft paint works fine to open the file. Note the number of pixels on each side of the bitmap (see the stretch image command in Paint). Use a calculator to figure out a multiple for each side that is still within the limits of the game. For the GigaUSA map, I multiplied the Cmapusa large map by 1.8 to arrive at a giga map (so it is nearly twice as big as the original).
The coast lines will look bad and the rivers will look even worse. Use a large foldout map to fix the coasts and rivers by eye (this is by far the most tedious part).
Also specific geographic features may come out crazy--bays, inlets, fjords, etc. They will all need serious retouching. And remember now that the scale is totally different, so you should tink about adding geographic detail that a large map doesn't have room for (other major rivers, tributaries, smaller deserts and oasis, etc).
Good Luck