So there you have it, Karthwasten is to be on the most western edge of Skyrim if you look up the map and Riverwood is about half way through the entire map.
though I might add that the most north western edge to the most south eastern one is a farther than just straight west to east.
From the look of it, I would have to say that Skyrim is probably a bit smaller than oblivion but I don't really care about that kind of thing because quality is much better than quantity and even though quantity can add to quality most games that have massive surface area don't tend to utilize all that area the best way they can, in all reality it's just empty space.
so in short I'll take Skyrim's way over oblivion's any day for sure.
I would also like to add a reminder that Skyrim has a lot of land that acts as a barrier and things of that nature that will cause you to trek around which will give you the pleasure of more adventure plus make the land feel much more alive and much more full filling.
PS. I love Bethesda game studios