A while ago Hideo Kohima showcased a map size comparison between Metal Gear Solid V: The Phantom Pain's Afghanistan area and the previous games of the series.

Today, during a Stage event, he clarified a rather relevant detail. The actual map is that vaguely trapezoidal shape limited by a somewhat lighter area, and not the whole rectangle.

Yet, it's still definitely big. You can check out the original image and one with the actual playable area highlighted in red below.