The maps and their sizes are within a .package file. The SimCity.exe only executes the packages and has some of the UI. It is very easy to mod SC, just make a package file and place it in the data folder, the game adds all .package files to the game. You would mod SimCity as you would with the Sims 3 because they use the same file formats and a similar mod loading system. I'm not a modder myself but checking the files called SimCity_RegionTerrain0.package and SimCity_RegionTerrain1.package could help with finding the plot border controls.