The art director of V recently said that Los Santos is as big as if you took GTA IV's map and smushed the all of the islands together.
Which, looking at the most recent city pic (the one from IGN) seems like a slight embellishment, the scale of the city seems to be what's gonna draw us in.. I don't think the city is necessarily large as much as it is detailed. It's about the size of Broker\Dukes\Bohan.
What makes you think that? Real life L.A. is bigger than NYC in terms of square mileage. I've heard that we can expect downtown Los Santos and it's surrounding neighborhoods (suburban, not rural) can be compared to the size of the entire GTA IV's map alone.
Just from checking a few of the pics we've seen so far, we've got a fair amount of areal\high up pics of the city. Though we've yet to see panorama I think we've got a good idea of what we're working with.
It's big, has lots of pretty lights and a grid-pattern.. But Just to me, doesn't seem bigger than The boroughs of LC.
The reason it looks that way is because Liberty City has more smaller streets than LS. 3 liberty city blocks are the size of 1 LS block. Just like in real life. The streets in LA are much wider while NYC streets are more dense and smaller. LS to me looks about the size of the entire Liberty City. It only makes sense.