I feel like with all these various landscapes, such as a forest, snowy/icy areas, a marshy looking area, and mountainous and volcanic areas, Skyrim will need to be larger than Cyrodiil. I mean, each area, such as the Reach, needs to be at least 1/3, maybe 1/2 ideally, the size of Cyrodiil.
This is a bit of a difficult concept to explain, so let me put it this way. There are around 7 or 8 different regions/landscapes in Skyrim. If Skyrim would be too similar in size to Cyrodiil, it means each of these regions will feel small, reducing the sense of size in the gameworld.
So, to account for each of the regions to be large enough, Skyrim needs to be at least 50% larger than Cyrodiil, but IDEALLY, at least 2x the size of Cyrodiil.
Any thoughts/feelings?
The terrain type plays a large role in the projection of landscape size. If you look at it, Cyrodiil was a decently sized region (though it wasn't exactly my cup of tea, I personally like the size and layout of Vvardenfell and the fast travel strider system in place) but it felt kind of unrealistic to be able to walk from "City x" to "City y" on foot in no-time, let alone if you are in a real hurry you can fast travel there with the click of a button. The fast travel system in Oblivion was less immersive than the Silt-Strider system of Morrowind. Having to pay x gold to travel somewhere through NPC interaction seems more realistic than clicking a city in a user interface and instantly ending up there. I believe a solid fast travel system (a system that implores more than clicking on a map in a menu) is one big step in the right direction to project a feeling of a larger open world.
Red Dead Redemption I believe really nailed this system to the post for a sandbox, but that's a little off topic. In sheer size, yes I would always love seeing a large world, so long as focus isn't taken off more important features, however it is a little bit more of a complicated matter than it may seem. Many other factors and the design approach of the world play a role in shaping a game world. Lore, geography, locations of interest, realistic scaling, and others, all play a role in how the world will be shaped in their design approach. I doubt when discussing Skyrim's geography the artists and designers start off by saying "Oh since we want Skyrim to be better than Oblivion, it needs to be physically bigger". Bigger might be better, but there are numerous other factors that compensate for size, and even determine size.