World Machine Dimensions:
300km x 300km
16000m Height
2017 Resolution
When importing to Unreal, I cannot seem to get the scaling right. What X,Y, and Z percents do I need to achieve a 1:1 scale?
Any help would be greatly appreciated.
Best thing to do mate is import the map at the default highest thats allowed. Then go into the top view make sure your in placement mode then middle mouse click and drag across your map. That will give you your map measurements in cm i believe.
Convert that number to meters or KM 
I’ll have a little dig myself see what options i set mine too.
Just to note however 300km is bloody huge!
Indeed. My current map is “only” 16km, and it takes 30+ minutes (of real-world time) to run flat out from one side to the other, not stopping for anything.
A 300 km map would take an absolutely massive amount of content (npcs/buildings/trees/monsters/whatever) to make it feel like it’s not empty.
But, to answer your specific technical question, a 2017 pixel map would cover ~2km at the default 100x100 x and y scale. If you changed the scale to 1000 x 1000, you would be at 20km, and your map would look rather bad (very obvious pixelation). Going even further to 10000 x 10000 would get you to 200km, but your map would be nigh unrecognizable at that scale.
This sort of thing is what UE’s World Composition and World Machine’s tiling feature are for. To make a ‘proper’ 200km map with tiling, you would have to export 100 x 100 tiles (i.e. 10,000 individual height maps), each measuring 2017 x 2017.
And we’re not even at 300km yet. 
It is an island so it will be surrounded by a lot of water as well. So, say I knock it down to 120km. What would I need then? And what would you suggest the tiled build to be for maximum performance? (?? x ?? Tiles)