I think whatt you want to hear and I may be wrong, but this really depends on what you determine to be your scale. 16 units in The Unreal engine is generally a foot. Then everything is based off that. It helps level designers use the grid and snapping effiecently and texures size at 256, 512 and so on are a multiples of 16 which is convienent. So what I am saying is the default plane created by blender could very well be 2km x 2km based on how you scale your scene objects.
I don’t really get the question, but the ‘standard’ scale in blender is 1bu = 1m so 2km is 2000 bu. You can make one plane that big and subdivide it - but it probably be huge before You get the detail You want. You may make 20x20 (400) planes each 100x100 bu. If You then set fog and camera clipping at say 800 most planes get culled from the camera view and the scene should render fairly fast.