Resizing GeoTiff as world partition

I have downloaded elevation map that is made of 52 GeoTiff files. The map that I have downloaded is about 800km X 600km but I want to resize it to a 20Km X 15km area and keep the height intact. I also want to convert it to world partition.

Every tutorial that I could find, explains how to bring a single GeoTiff into Unreal and as a single map and are mostly made for UE4 that didn’t have support for world partition.

I tried a few paid plugin but didn’t produce the desired outcome. Either they add their own objects and layers into project and add original GeoTiff too, or creates a single landscape which then importing the second GeoTiff replaces the first one.

I even tried Cesium, either I don’t know how to use it or it also adds objects into project and can’t resize the landscape to the size I need.

Can someone point me to any helpful document or tutorial that shows

1- Details on how world partition works with landscape image
2- bring multiple GeoTiff into Unreal as chunks to create a single landscape.
3- Resize the landscape to the size I like

Thanks in advance