Map scale
The key.
You would look at the map key.
The map scale is usually located in the legend box of a map.
Maps are flat and Earth is round
You multiply by the scale factor. For example, if the scale is 1:100,000, you would multiply by 100,000.
The shortest 'theoretical' air distance from Washington DC, USA to Manila, Philippines is 8572 miles. Actual distance would depend on airport location and actual route of the flight.
if you really wanted to know the actual size you would go see it yourself you lazy people!
The straight line distance would be 350 killometers
According to the Northwest Airlines Mileage Calculator, the Distance from Detroit (DTW) to Madrid (MAD) is 3,989 miles. I assume you wanted air miles and were not going to use an amphibious vehicle otherwise I would have given you nautical miles.
I would assume that you would use cm or even mm if you wanted to be more accurate
No. Brighter distant stars can have the same apparent magnitude as fainter stars that are closer.(Absolute magnitude does not refer to actual brightness, but rather to what the brightness of a star would likely be at an arbitrary distance of 10 parsecs, rather than its actual distance.)
The straight line distance would be 350 kilometers (3.5 x 100 km). Multiply the measured distance by the scale.