GTA V's new map was unveiled for the Cayo Perico DLC. The trailer for The Cayo Perico Heist appears to show the new DLC location is an island that is not apart of the original map.
GTA V New Map Revealed for the Cayo Perico DLC
The size of the island is hard to tell. Based on the number of locations shown in the trailer, however, the island looks massive. The trailer tends to show a lot of beaches and docks. Based on the appearance of a submarine and speedboats, it's a safe bet to say that there will be a lot of gameplay around the edges of the island.
The trailer also showed some locations more inland. A plane flying overhead signals that there is likely an airport somewhere. There's also farmland on the island near what appears to be a radio tower. The trailer also shifts our focus to what appears to be a heavily guarded estate. Considering the name of the expansion, it's an estate that players will likely have to break into.