During the latest Active Time Report Broadcast, Final Fantasy XV Director Hajime Tabata clarified previous statements about the size of the world of the game.
Tabata-san mentioned that the area in the demo is just a small part of the Duscae region. The whole region will include some cities and towns and even old ruins in the full version of the game. The whole world of the game is about ten times the size of the whole Duscae region.
Of course this doesn’t fully clarify the size of the final map, since we still lack one part of the equation. Tabata-san (probably intentionally) didn’t reveal how big the whole Duscae region really is. We just know that it’s much bigger than the area included in the demo.
Tabata-san also explained that there aren’t any big towns in the Episode Duscae demo, but there are many in the final game. The demo will instead have little outposts populated by interesting characters, with whom players will be able to interact.