Right at the beginning of the game, you will be able to set up the size of your map. Don’t be a megalomaniac though, it’s better to start small and expand with each play-through.
Also, the game might start lagging if your city gets too large. The terrain can be adjusted to your needs. Terrain elevation can also be adjusted. Then, there are some buildings that you might build. You can also build on water. The most important thing is to have a flat terrain base.
Don’t forget to build roads connecting the buildings so that cars can drive there. It’s an interesting online game, where you can build up the whole city according to your imagination, so come try it out!