how to mod city size, simcity 4

The trick is to modify your config.bmp in your cities folder... Go to your cities folder in My Documents/Simcity4/Regions/ then to your desired region..

Open one of them, and you'll see a file config.bmp. Open that file in MS Paint, then simply alter the pixel size to 32x32.. (Really big) or you can also use 16x16 for a smaller region and use this colour to paint the image: (Under- Colors>Edit Colors)

R:0
G:0
B:255

With those values inserted, you'll get blue, so simply paint the entire 32x32 or 16x16 tile with blue. That's all! So when you enter into the game, open up that region and voila!

But be warned: It's better to start an empty region. The reason that it could possibly corrupt other cities (files) in the region and the game might freeze. So start an empty region with no cities for better safety.