The map of the game has leaked (right-click to view in full size):



Here's a comparison with the map of its ancester, San Andreas:



Also, a comparison with the map of its predecessor, GTA IV:



As you, it's tree times bigger than Sandreas and five times bigger than GTA IV.

On the GTA forums they've made a replica with more information on landmarks and such:

http://gtaforums.com/topic/491242-mapping-los-santos-buildinglandmark-analysis/