Being practical, POVray can render accurate images of
values beween 0.001 and 100k POV units, and so the
scale you choose should be picked to fit your scenes
values into this range. I prefer 1 unit per inch,
1 unit per foot, 1 unit per 10 feet, or 1 unit per mm;
but you can always combine them with scaling.

In particular it's difficult to fit spaceships and planets
into the same scale because of the large disparity in
size. In these cases you need to calculate the aspect
ratios and place scaled planets closer, so they still
appear on the screen as expected.

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