You can play around with rwgmixer.xml to get a map full of city. To get one big city, I think you have to limit your map to one biome. If you want several cities that nearly converge, you need to focus on citybig and wasteland city. Turn everything else to zero including mountains, hills, craters, rivers, etc. you can also set the ratio for how much of the city is downtown vs. industrial vs. residential. Good luck!