Use world composition. Use engine versions that work (.18, .25 and thats about it).
The new engine may as well not exist for how well it works.
Who’s “they”? Id1ots? They tend to say a lot of silly things. Like this.
“Large world coordinate” also means nothing btw. It’s probably dubbed by some egghead that couldnt understand the difference between an integer and a float or something. They 
hiring these types of people at epic… Really it’s the reason why the engine is in the state it is… the 4 great people they have on stuff are simply not enough to supervise the mess their superiors decide to make. But i Digress.
Its either Double, or Float. And the only difference the precision makes is that when you look at an object about 30km away from you it could potentially look slightly bettter.
How much so is already arguable.
Back to your ask:
20km^2 is literally nothing.
Even without following best pracrices you should be able to tile up a set of maps to cover the area.
Best pracrice wise, you probably want slightly bigger tiles. Around 2km each is probably better.
The docs on landscape optimization will give you epic’s approximation of what should in theory work best. Keep in mind its a theory.
And, “create” has nothing to do with “able to run”.
You can create a 300km^2 world with import batches using a tile size of 1km.
That doesnt mean anything you put inside the computer is going to be able to display and get the end result to work given the sheer amount of drawcalls requires to render if.