Never understood the concept of measuring map size in km. It's like measuring distance on a map while ignoring scaling. Something like source's hammer units make a lot more sense to me.
1 unit = 1cm in UE, so 20km has some value as a measurement in game.
Of course, what matters more is what you do in that 20km, but it's also a limit coupled tightly with distant mesh performance and float accuracy issues.
