[DaftWager]

I'm going to assume you mean a new blank region, and not a previously built up one.

In the game, create a new region using whatever name you like. It can be plains or water, it doesn't matter.

Exit SC4, look for the new folder for the region in My Documents\SimCity 4\Regions (or whatever you have set it to if you changed the default)and copy the new config.bmp into that folder.

Go back to SC4, the original blank default region with whatever name you chose will come up, go choose another region, then come back to your new region in progress. You should see the effects of the new config.bmp (if it is different than the default) in regard to city sizes.

Now while still in the region window, press Ctrl+Shift+Alt+R. Navigate to wherever you have the greyscale height map for your new region, click OK, and wait. If your computer is slow, or the region is very large, wait a very long time!