I agree. Judging from The Athasian Cartographer’s Guild, as well as the other “Dead Lands of Athas” supplement by Gerald Lewis, I would estimate our area of interest is about half again larger in both directions than the Tyr Region Tablelands map from the original Dark Sun Boxed Set.
If we were to compare it to the map you just posted, I would very roughly estimate the width would equal to distance between the west edge of the Valley of Dust and Fire to the western edge of your map, and the height would be equal roughly to the distance between the Obsidian Fortress and the top of the Troll Grave Chasm. While there is a lot of black there, it does wind a bit across the map. Personally, I think the Cartographer’s Guild made this area a bit too big to be any fun for game purposes. Besides, it’s kind of a creative cop out to make this as big as the rest of the known world put together.
I’ve made a development folder, where I’ve taken the Cartographer’s Guild maps, and made a composite on their unrolled planetary map of all the known maps we have, so that gives us some idea of the layout. Have a look if you like.
Does that help? Drop me a message if you’re willing to discuss. I teach Photoshop and Illustrator, so I might be able to help make this a bit easier for you: