5.5k by 5.5k would be good, and with that I think a restriction to how many vassals a nation can have would be great, seeing as most of the plains in the center of the map are clogged up by outdated, dead oren estates - all that land could've been utilized by settlements and lairs that would actually be active
While this does indeed look really fuckin cool on paper, in game this would just be repeating mistakes this map made, none of this beautiful and scenic land can be used in any way, and traversing it is nigh impossible