Good question about the size of the map. I don't think it really matters though, as CC3 is a CAD program. Meaning objects will scale and you can export maps at larger dimensions without quality loss.
I always start my city maps at the default 1000 x 800 pixels. If you want a small village you can place larger objects and you will run out of room quickly. Or if you want a large city, make the objects smaller and you will have a lot more room. Meaning I might place a size 5 tree in the village map, but place a size 1 tree in the city map.
Check out my city tutorials if you would like (link is in my signature). It will also explain how to add dirt patches (which can also be used to create a beach).