The map isn't entirely accurate, but it gives a bit of an indication of just how big the game world could be. Though, we have to bear in mind that San Andreas was a whole state. Check out the links below to see for yourself. We'll leave you to form your own opinions on how big you perceive the map to be.
First we used Google Maps editing kit and dropped in all the known locations that have been seen in the trailer. Then we added the Liberty City/NYC boroughs revealed by the Game Informer preview. Finally we drew a boundary around certain geographical locations that we can imagine serving as useful boundary points in the game - sea, major roads and the JFK airport for example.
The results are pretty surprising. The area the map could potentially cover is huge.
Links: GamesRadar article | Liberty City on Google Maps