How thick in ground units is this line on a printed map?

One PostScript point is one seventy-second of an inch (1/72"). One inch 0.0254 meters. So one point is 0.000352777778 meters. Multiply this by the denominator of a map scale to get the scaled size:

[scaled distance ] = [line width in points] * 0.000352777778 * [map denominator]

By this math, a 0.2 point line is equivalent to 1.76388889 meters on a 1:25,000 map.

It should be noted that on most printed paper, ink will bleed away from the paper fibers to which it was initially applied. So while you might set a line to be a specific width, it will be slightly larger when printed. In typography, ink traps are used in some typefaces to counteract this effect, especially with small fonts and/or when printing on bleeding-prone newsprint paper.

As per dmhar's answer, here's an automated solution that you wanted. Since I don't know much about typography or maps, I have no idea what are the commonly used line widths and map scales. If you can provide me with a list, I'll add it to the page.