Nobody knows the exact size of the station as portrayed, not even the people who designed the shooting model, or shot it, or directed the shots. Estimates vary from a vague original design intent of about 900 meters to up to two klicks of diameter. Certainly it would be impossible to decide upon a figure that agreed both with window sizes (the ones on the Promenade are a nice telltale) and the size relative to docked starships (the worst offenders are the big Galaxy and Nebula ones, but the Defiant and the runabouts also produce their share of problems).
The exact specs of the shooting model should be available somewhere, though.
Timo Saloniemi