http://www.wisegeek.com/what-is-the-difference-between-a-coupe-and-a-sedan.htm

Quote:

sedan has little to do with doors and are made clear by the Society of Automotive Engineers (SAE), which standardizes definitions of the different types of cars on the market. Measurements of a coupe and a sedan are as follows:


Coupe equals less than 33 cubic feet (.934 cubic m). Sedan is equal to or greater than 33 cubic feet.