Grand Theft Auto 5's map is huge. It's a fact that comes as little to no surprise to anyone who's been following Rockstar Games' progress over the years. It's bigger than any of the previous games in the series, and that's just what we've seen of the game. It spans from the ranges of Mount Chiliad all the way down to whatever the name of the bay area is in Los Santos, whereupon you can opt to go diving out into the open sea for even more vast locations to explore—underwater.
Heck, the area in the game is so big that Rockstar has described it to be five times larger than Red Dead Redemption's map. According to the studio, Grand Theft Auto 5's Los Santos map is bigger than Red Dead Redemption, San Andreas, and Grand Theft Auto IV combined. As I mentioned before, there's wilderness, a military base, and even an ocean floor for you to explore underwater. It's just that huge.
To give you some sense of scale, a redditor named C-Ron has underlaid fan-made version of GTA 5's map beneath those of previous games in the series to showcase just how large the game actually is. If you shoved everything to one side, you could find room to fit in Red Dead Redemption's map, and then some.
Keep in mind that the map scale is just an estimation of how big the game's area actually is. Until Rockstar themselves release an official map, all we can do is speculate as best we can.
