Soo..how big is it ?

Approximately 15km squared.

Wow, I didn't know that, it seems smaller then 15 square km. Probably because Oblivion was 16 square km and seems so much bigger then DC.

I think oblivion is advertised as 16 square miles (not kilometers) or roughly 4 miles per side. Above ground of course not counting dungeons, Shivering Isles, Oblivion planes etc.

And I think Fallout 3 is advertised as about 3/4 the size of Oblivion, so about 12 square miles. Again not counting DLCs or "Dungeons"

Since I live in the DC area, it's really obvious how they compressed the distances. The real DC area is much, much larger.

No, you live in Kvatch, remember

From Kvatch. I'm in the DC area at the moment.

It would take a long time for me to run from Fairfax to Germantown. But it doesn't take my character all that long.

EDIT: Just checked, it's about 33 miles from Fairfax to Germantown in real life. So it's a much larger distance, but then you don't need to fight as many Mutants on the way in real life.