I model to *exact* sizes in feet, because my ships have interiors, and are meant to be used with other figures etc, so I must have exact scaling.
For objects, they will often be used by human figures, so again, scaling is vital.
I use Rhino, and render scenes in Vue6 Infinite (which has proper metric or imperial scaling, out to many kilometres or miles if you wish).
Scaling also affects realism. Atmosphere and ligth has certain reactions we instinctively know the look of.
Space shots can avoid some of that. But not interiors or normal scenery.
I also do cut aways and layouts to explain my ships and for gaming
http://www.silverblades-suitcase.com..._layout_26.png