It' s official, Los Santos is indeed BIGGER than LC from GTA IV, but it' s not as wide, which means it wont be necessary to get through shi* load of bridges which will save you some time. Here are my (I believe 95% right) measurments:
So the whole landmass is what? More than 5 times bigger than Liberty City?
It' s hard to say because LC is so difficult to connect into one island. but yeah I' d say it's about 5x smaller than the whole SA in GTA V.
Edit: And I' m only counting flat landmass, you have to remember that the mountainsides make the map even bigger, because it' s not just flat space, plus the underwater. OMG this game is gonna take my life away from me.