The online journal of John Davidson, who just so happens to be the Editor In Chief of the Official Playstation2 Magazine is exploring Rockstar's claim of SanAn being multiple times larger than any/all previous GTA title(s). He explains that the cities won't just be connected with roads, but with massive suburbs each potentially the size of GTA3. The quote is pretty much self-explanatory, yet should give you an idea of how ambitious our beloved game developer is.
“So... apparently the much-talked-about "in between bits" of San Andreas, which for want of a better word are the game's "suburbs" are each apparently the same size as the cities in GTA3. It's not just a few roads joining Los Santos, San Fierro, and Las Venturas together - but instead there are three enormous communities in which you can wreak just as much havoc. Imagine the possibilities... think of Thelma and Louise style cop chases with squads of cop cars chasing you across the desert landscape, or Dukes of Hazard style shenanigans as you try and shake off the law while hurtling through sleepy little towns.”
Sources: Gouranga!, 1Up Blog of John Davidson