The terms “size” (in vertices) and “resolution” are used interchangeably. Heed the “recommended” sizes on this page:
You can enter those sizes in the WM project settings. If you want a specific size map, WM will give you a size (KM) based on your resolution and scale (pixels/m), and from that you can multiply the X, Y, and Z on import to UE4.
Follow these steps closely:
Note the RAW and r16 file types.
Best to learn by experiment with smaller scale maps.
For tiled landscapes, it is best (AFAIK) to use World Machine. Importing tiles from your own toolchain will produce artifacts.