There is NOTHING on the drawing that you can guess at the size of and be within a factor of 12?
If the drawing was made to millimeters vs meters, that should be discernible since it is a factor of 1000, likewise inches vs feet but that is only a factor of 12. Even millimeters vs inches is a factor of 25.4.
In the US, maps are often drawn in feet. Elsewhere it is likely to be SI units.
Is it a 3D map that shows elevations? If so, go to a side view and look at contours, if the contours are labeled you can measure from one to another.
Do the layer names offer any clues? Sometimes contour lines are on layers that include the elevation...
Given that this is a map it may follow some convention - where did you get it, who drew it?
If nothing else works, go back to the original drafter or organization, they should know.
You can type "Units" or "DDUnits" on the command line and it will tell you what the units are currently set for (but this can be changed anytime and may not reflect the data).
You can try File-Drawing Properties and see what if any information is there. Probably no help...