1 Motti: This battle station is the ultimate power in the Galaxy! 120km in diameter, with an inhabitable shell 2km thick...
2 Motti: We have space for 700,000 troops, not to mention countless bottomless chasms spanned by narrow walkways with no guard rails.
3 Stormtrooper: Lord Vader, we've found the rebel base!
3 Vader: Where?
3 Stormtrooper: It was here inside the Death Star all along!
4 Stormtrooper: We weren't patrolling that 875 billion cubic metres. I mean, it's only 1% of the habitable volume. We just missed it.
4 Vader: You were saying?

Thanks to Steven Ehrbar for pointing out the implications of the arithmetic behind the Death Star. And yes, those dimensions are canonical.