Elswyer and Black Marsh are smaller provinces, they would be perfect for a spin-off by Obsidian.
I disagree on all acounts. Black marsh and Elsweyr have other characteristics that make the size on the map irrelevant. Once they decide to make a game in these provinces I assure you they won't be smaller in square miles gamespace than Oblivion or Skyrim, because any sane dev won't say "let's make Elsweyr smaller and even more cramped than Oblivion because that's how they appear on the map so we must stay true to the original map". The size of the map and distances between towns and world design and character speed and city scale will always be adapted to the current resources and technology, to the amount of content the devs can pull off in the current circumstances, not to a stupid map that was once drawn to give us an idea about how the continent looks. I also hope that the actual Black Marsh game will feature much more threads of water than what appears on the maps I've seen. If it makes sense for the game, if it serves the gameplay, the map (and the lore) should always bend to make way for more content and better gameplay.
Also, a big no to Obsidian. Screw them, Fallout was something different, it was their game in a way. I hope single player TES will always stay with Bethesda, no non-Bethesda spinnoffs.