Here is something to get your juices flowing. Visual comparison done by someone on Reddit to show the map size of GTA5 compared to GTA4, Red Dead Redemption, and GTA: San Adreas. I must admit this game is gonna be MASSIVE
Here is also a link to the reddit thread showing all the math and calculations that was involved.
http://www.reddit.com/r/GrandTheftAu...n_the_other_i/
