During the latest episode of the webcast Kojima Station legendary Metal Gear Solid creator Hideo Kojima introduced then map of Metal Gear Solid V: The Phantom Pain compared to the size of the maps of all the previous MGS Games.

You can notice the silouhettes of the maps of the previous games in different colors at the bottom of the picture below. As you can see the map of the new game is enormous compared to earlier installments of the series.

The map also includes a ruler showing that it's 4 Km wide. Unfortunately the other side is blurred, so it's impossible to say for sure, but it seems over 6 Km, for a surface of over 24 square kilometers. In comparison Skyrim was about 38 square kilometers.