Yes, factor in the land mass and the new underwater environments, GTA V is five times the size of Red Dead Redemption's world. To put it in another way, Los Santos and the surrounding area is bigger than Grand Theft Auto: San Andreas, Grand Theft Auto IV, and Read Dead Redemption combined. Best of all, the entire map is open right from the start. Oh yeah, you can explore any part of the map as soon as you start up the game without any repercussions from the law this time.
The new map is humongous with a bunch of varied environments, and there's going to be a wide assortment of activities you can do to fill in all that open space with. Hunt wild animals, go base jumping, hijack security vans, play golf, do yoga, buy businesses, scuba dive in shipwrecks, and so much more.
Check out my preview of Grand Theft Auto V to learn a ton more new details about the open world, combat, the new missions structure, plus a ton of other details.