In Oblivion I pretend the scale is larger and one area has a bunch of cities and is inaccessible. All the urban areas depend on the dangerous country side areas for food and trade since they have an abundance of food and raw materials. All in all, I just pretend the world is actually a lot bigger, Im just in the most important area of the world where all the major resources and landmarks are.
Daggerfall was 63,000 square kilometers but had randomly generated areas. They never did this for Oblivion or Skyrim, I figure once you have the game engine and all its features, it would be easy to create variations of it to randomly generate. To bad no randomly generated area mod has ever come out.
I would say population wise during the timeline of Skyrim. These numbers I made up only include citizens and not outlaws or bandits or secluded people who have their own small domain away from the world like renegade wizards or Marauders/Vampires. They could number 400,000-1 million depending on the province.
Skyrim: 5,960,452
Cyrodiil: 10,820,452
High Rock: 3,542,132
HammerFell: 6,421,543
Summerset Isles 5,520,421
Valenwood: 6,845,421
Elswyr: 3,215,632
BlackMarsh 4,860,425
Morrowind 4,525,842
http://forums.uesp.net/viewtopic.php?f=30&t=19903
My estimate for Cyrodiil was based on this, assuming the last post is Legitimate Lore wise. Is it? He said "intended".