Well, you'd have to shorten routes on the World Map, which doesn't effect the actual overworld rooms, and it also gives you more world map room for more cities. (Or a region, in your case :D)
Look at screenshots of the World Maps in Gaia, Unbound or Light Platinum and create a map working off of one of those. It has to be the same size as the original Firered map, (I'd recommend Firered for this) or the map will stretch outside of the screen, with no way of seeing the rest of it.
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Did you see the tutorial? It shows you how to edit the world map, and insert your own.
Just an example, if you wanted to go in Pewter City, but you wanted the World Map to put you in Cerulean, use the World Map Editor in Advance Map 1.92 (1.95 has it disabled)