It will be huge.
ED is pretty much on the border of Duscae.
We know that Lestallum is in the Cleigne Region and it's not too far. Technically, I'm not certain that the Disc is part of Duscae, either.
I saw there was a Duscae Junkyard as a driver marker in a vid a few months back. Still haven't seen a lot of that Region.
If the Region is 5 times as big as the Demo - then ED is only 2%. Hard to get your head around that!! This sort of scale is really going to compliment the Road Trip, the sense of adventure and the overall theme of a Journey - in my opinion.
Right??? I am a firm believer that we are seeing roughly 20% of the map right now. If there are 5 main kingdoms or major locations; Lucis, Tenebrae, Niflheim, Solheim (or whats left of it), and Accordo, then the map of Lucis (what we see in the first 4ish chapters) isn't the only area where this game takes place. They never show that path or river that is exiting Altissia, which I'm sure leads the rest of Accordo (they only show Altissia from the front, not the other way around), just as Insomnia leads to the rest of Lucis.
Also, Kingsglaive shows almost exactly what is north of the current Lucis map:
http://i.imgur.com/BjD9SH1.jpg
If you look closely, the layout of those roads are scaled and organized in a similar way to the map we currently have. If you look even closer, you can actually even see the top of the in-game map roads actually connect with the bottom of the movie map. There are definitely other open world zones at the least.