I started planning it using the free GoogleSketchUp, an easy to use
3D modelling program.
This helped me to visualise what the model would look like before I even started building it.
I then decided on a scale that would be practical
for its overall size. N scale, 1:160, was chosen as people, cars and buses were availabe for purchase in that scale. The model would then be 800mm long x 400mm wide overall.