Physics grad Matthew Sutton wrote a very interesting analysis of Skyrim's size on Quora, and if you've explored Bethesda's grand, expansive game world, you'll probably be surprised. Extrapolating from the height of NPCs and the buildings they inhabit, Sutton estimates the world to be (on the high end)... 3.7 miles across from West to East, and 2.4 miles from North to South. So like this:
In real world terms, assuming Sutton is correct (and his calculations seem plausible), that means Skyrim is much smaller than San Francisco, which is roughly seven by seven miles across. Look:
Pretty striking for a 3D simulated world that is so immersive and has so many places to explore, some players spend much more than 100 hours discovering new things within it. Interestingly, an academic did a size comparison of San Francisco to the virtual world of Second Life in 2010, and found that SL's total landmass was much larger.