The map size of Ground Zeroes in Metal Gear Solid V will be about 1/300 the size of the area found in The Phantom Pain section.
This was revealed by Kojima in an interview with Japanese site Game Watch, and translated by NeoGAF user raven777.
Kojima has said before the areas in The Phantom Pain will be much larger, with Ground Zeroes serving as a prologue to give players a chance to adapt to the new open world aspect of the game. But this seems to suggest the areas will not only be larger, they will actually be huge. It is unclear wether Kojima is talking about individual areas here, or all locations added together.
Source: Game Watch, NeoGAF