Keep in mind the free version of World Machine can only export a height map with a max resolution of 1025x1025. So if your land mass is 1km x 1km, that’s a resolution of 1 meter per pixel. If you want a 10km x 10km land mass, that brings your resolution down to 10 meters per pixel.
I haven’t had any problems with texturing and changing size in UE. I typically have to adjust the height scale because UE tends to squash the height map on import.