When I decided to try my hand at 3D modeling, and printing parts that I could use on my models, I started by making a 14“ Pyle-National headlight, only because I could find a decent-enough diagram for it.
After printing, I began seeing how it would look on some of my models, but compared it to headlights on them and to drawings in the Model Railroader Cyclopedia (which are in the same scale), and found that much of the time, it looked quite small.