Executor-class Star Dreadnought = 19 km long. Its approximate 2D area (not surface area) is much greater than that. By my eyes and a top view of the Executor from the scene where the Executor plunges into the Death Star II, I'd say that the ship is maybe... 4.5 km wide. Sound good? Adjusted for an approximate triangle, that's 19 * 4.5 / 2 = 42.75 km^2 of area. So approximately one and a half Executor-class Star Dreadnoughts in area.