I believe its a combination of rdr, and gta four if so this may give us an idea, but the streets look too broad in sa and the map looks shrunken in LC http://www.overclock.net/t/1158206/official-grand-theft-auto-v-information-discussion-thread/180
and this was said on another site: "Just how big is this world? Art director Aaron Garbut crunched the math. When you include interior and exterior spaces together, Los Santos is bigger than the worlds of Red Dead Redemption, San Andreas, and Grand Theft Auto IV combined, with room to spare."
Now that is BIG!
Coming from someone who won't get to enjoy the game until way later... o well i'll get the game free thanks to my survey sites!
**Self Proclaimed Official Bro-Hugger of the Xbox Boards, Total Bro Hugs given: 5**