[neckro23]

In Source the scale is fixed (1 "unit" ~= 1 inch) and I imagine the same's true for UE.

[antidamage]

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.