Area 69 belonged to Las Venturas. GTA V is taking place in Los Santos - the difference is though that it's a reboot, their re-imagined Los Santos. Think GTA III to IV. GTA V is going to be Southern California, with the surrounding hills, countryside and all that jib-jab. So there will be Los Santos, along with possibly a recreation of San Diego and other cities.
So it is going to be very big, not one small island like Los Santos in GTA: SA. Forget about that one.