Technically, the screen size is 7" diagonally. But just to be picky...
Because of the system bar at the bottom that contains the back, home, and recent apps buttons, the usable size for most apps is 6.75" in portrait mode and almost 6.875" in landscape mode. However, media players (at least MX Player and BSPlayer, which I use) will draw over the system bar to utilize the full 7", depending on the aspect ratio of the video.
Additionally, the notification bar at the top of the screen also makes the usable screen size slightly smaller, though many apps draw over this area to maximize screen space.
And that, my friends, is the anal version of how big the screen is!