This eluded me for a long time. It’s based on Resolution * Scale.
A map with a resolution of 2048 * 100 scale = 204800cm = 2.048km.
So any of these = 8x8km:
2048 * 400 scale
4096 * 200
8192 * 100
Also in Top-View, dragging middle-mouse will act like a ruler, showing cm. (100,000 = 1km)
World Machine may be scaling for 8km world but if you output a 2k image, it has to be scaled up 400x to actually be 8km. It’s a mind bender.