Thanks for the answer!
The DEM I have has 10 meter resolution, so the terrain will certainly be quite rough. But I am using it to illustrate a large national park, so it is not crucial for the terrain to be perfect down to every meter. What is more important is that scaling is correct in relation to width/height. Naturally I would also like to keep the 10 meter resolution in unreal to get as much detail as possible, and this is where I get confused.
Since my heightmap is exactly 5041 x 5041 pixels, I guess thats the maximum resolution I have available. The heightspan in the files goes from 0 to 1900 meters.
Until now I thought UE only accepted heightmaps in the exact sizes given in the TchnicalGuide (8129*8129, 4033x4033, 2017x2017 etc), but the 5041x5041 map was accepted. But with autofit my terrain’s overall resolution is now 5101x5101
Another issue is naturally what scale I should be using to achieve the 1900 meter height?