Well, about 5-6 the times of Vice City for land, but including water who knows.
This is actually a very good theory. It would be like having Vice City and III with San Andreas graphics and physics. Like Urbanoutlaw was saying though it would be a huge pain in the ass in terms of time consumption. But it would be very well worth it, just imagine MTA:SA:VC:LC or SA/VC/LC:MP!