1 uu = 1cm.
Changing the setting you mention will not do anything. None of this will help in UE4. You’ll run into floating point errors early on. You will get a stable simulation 2.5 km from the world origin, things decay fast after that. At 5km it gets really shaky. You can’t spawn actors further than 10km from world origin, or thereabout.
Try this, see what happens:
You generally fake things. Is the closest star really 150aus away? You tell the player it is, put a marker and a label on the screen → there, a star. They will not spend the next 400 years trying to get there. They’ll activate the hyperdrive and whatnot… But you probably do realise all that already.
You could scale your world down. Does the spaceship need to be 100m long? Perhaps 10m is fine, perhaps 1m is enough providing everything else is to-scale. The player won’t notice.
Now that everything is 100x smaller, you can have a space battle in a fake 250km radius. The universe map will not be the size of the universe, surely, it would be a model:
Also, look into large works and world shift origin: