You can use the grid to determine the size your terrain.
Make the recommend 4033 setup and then switch to ortho top down view and you can see on the grid that it is just about 4km.
If you need to fine tune for meters you can adjust the scale slightly or change the landscapes component settings to something otehr than the unreal recommendations.
In some cases, precise terrain size does in fact matter. I agree it is annoying when people try to redesign your game when they know nothing about it.
And the unreal landscape technical documents are obscure as hell, but the great thing about digital work is that it is effortless to test things out so we can also encourage noobies to just punch in numbers and look. It’s easy to do.