The "scale" should always be the same, assuming you're modeling North American/European railroads, at 1:160, 160th the size of the real thing. HOWEVER, the real thing comes in many sizes. In the mid 1800s, North American passenger cars were often only about 50 feet long, which would be 3 3/4 inches in N scale. Most 20th century cars are 80 to 85 feet long, or 6 to 6 1/2 inches. European cars can show a similar range, although I'm not sure about their exact sizes. American freight car models can range 2 inches for beercan tankcars and ore cars to almost 7 inches for "89 ft" flatcars.
The easiest way to tell how long the car is is if the scale length is given, such as a "40 ft boxcar". Since I don't know where you live, I'm assuming it's in North America, and you use inches. Ten N scale feet equals exactly 3/4 inch, so converting scale length to actual length is easy, as long as you work with whole numbers. You also have to allow for couplers and the fact that freight cars are usually labeled with the INSIDE length, so a "40 ft boxcar" as actually about 41 ft long, and the couplers stick out past that, so the N scale model would be about 3 1/4 inches.
European cars are usually label as "length over buffers", and the same 1:160 scale factor applies, although you may need a calculator. Divide the scale length by 160 and you'll have the actual length, if the car is properly scaled. In practice, the buffers are often shortened, so the cars won't hit on sharp curves, and like American models, the couplers usually stick out too far, so adding a few millimeters would be expected. A 10 meter car should be 62.5 millimeters long. For a very rough estimate, the scale length in meters times 6 is about the model length in millimeters.
