I know at the bottom of a CityEngine scene you have the option to display axes, information, and a compass. Is there an option at all to add a scale bar? Or is this not possible because of the 3d space (that is, when viewing from the 'in-person' ground level, not from above like a map)?
You can display the grid (drop down arrow next to icon in viewport toolbar), and the information display will tell you the current grid size. The grid cell size changes as you zoom in and out.