It's los santos only. It sure as hell isn't going to be small though lol. From what I've read, the city itself will be around the size liberty city was (or at the very least somewhat the same size). Then you have the outskirts and the giant lake and mount chilliad and surrounding forests and county towns all across the map. Biggest and probably the most ambitious map R* have created.
I r Ninja. I eat brains. NOO! I DON'T WANNA EAT A GUITAR!!
Liberty City isn't an exact copy of NYC. It copies districts and landmarks, tries to stay generally true, especially to Manhattan, but it's a fraction of the size. Los Santos will be the same, going by screenshots and what Rockstar has said.
I don't think the size is something to worry too much about. The city alone is still going to be bigger than what most GTA clones offer.