I suspect the real answer is that "beauty is in the eye of the beholder"- find a sample among your cars and models that you like, and measure it. The easiest method to measure something like this, in my experience, is to use a feeler gauge (as used to set gaps in spark plugs)- it is easier to find and fit the correct feeler gauge "blade" than to try to measure a gap in thousandths with a micrometer or caliper (for me, anyway) and they commonly have blades as thin as 0.0015 inch.
What will make more difference than width and depth may be the degree of weathering. My own opinion is that very fine lines (scored with the back of an Xacto blade, for instance) painted and given a light wash of india ink or similar thin wash (which will settle into the score lines), will read as more realistic than "gaps" of several thousandths wide. So the answer here, in terms of 3D printing, might be "as small as allowed by the resolution of the printer."
I can already see that I will probably spend an hour going through tool boxes tomorrow trying to find my feeler gauge so I can measure the board spacing on my cars.
