Start small! Do the natural evolution of hamlet > village > town when it comes to size and the layout, otherwise you will run out of cash if you start planning that megalopolis too early. The big city will come eventually, especially once all of your towns begin to merge into one larger city with each tile being like a different region or district.
And think ahead when it comes to sewage, my first town is polluting the main water source of the next tile I want to expand to because I never thought ahead about expanding into that area. Now that I want to, the citizens will be drinking crap if I set it up!