Creating new regions in SimCity 4 - very nice article explaining how to create large tracts of land quickly, and then how to lay out custom regions. The basic summary is as follows:

Regions are cut into the various city sizes like a jigsaw puzzle consisting only of square pieces. A small city is 1km per side, medium city is 2km per side, and a large city is 4km per side. (The default size of the map created by SC4 when you first open the box is 16km x 16km.) You can make regions any size (20x10, 10x10, 8x17) but he recommends that you keep the overall area around 256 sq km (the initial default). Don't forget that you have to then chop it up into city "squares" when deciding. One of the areas that I'd like to map is going to be 16km x 24km, which is a little larger then the normal sized region.