Terrain Editing Tutorial: Terrain Skybox Size Configuration for Farming Simulator 17 (FS17) by PMC.
If you are grateful for all the work PMC has done in the past 25 years, use Support PMC page.
HOWTO configure FS17 terrain skybox Size.
I took the maps/sky/ directory from your sample terrain and edited maps/map01_environment.xml to read >environment filename="maps/sky/ instead of "$data/sky/
Open sky_day_night.i3d file in Giants Editor FS17, select sky transformgroup, then change scale X, Y and Z to 8. Do the same for skyUS_day_night.i3d file too. Now your skybox is enough for 8.1 kilometer x 8.1 kilometer terrains as well. For PMC 32768 Terrain I made the scale 16.