Well world scale in UE4 is 1 UE unit = 1 cm so a kilometer = 100000 centimeters.
Soooooo
1 mile = 1.609344 kilometers.
In concept though 3d apps have no clue as to scale except for sizes relative to each other with in 3d space until you tell what app you are using what the world scale is based on a unit. In UE of course this is a fixed vale of 1 unit = 1 cm but in most if not all 3d editing applications you have to tell the application what the world scale is.
Since 3ds Max is my application of choice I can tell Max what a unit represents be it 1 inch, 1 millimeter, 10 miles or 1000000 kilometers and then set whatever form of measuring tool, say the tape measuring tool, to what ever conversion of the world scale I wish. In this case since I’m working with UE4 my default in 3ds Max for world scale is set to 1 unit = 1cm.
So with this set up lets say I want to do a true to world scale of Mount Everest I just go here
Set my measuring tape to meters and scale up until the model is 8,848 that if I want to convert to landscape as a proxy.
That’s of course if you want to make a real world scale version of Mount Everest and anything else could be relative scaled to the player camera or character model.