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Terrain Size Change FS17
FS17 terrain size is defined in the main TERRAIN.xml config file, ie "map01.xml" by default. You are not forced to use 2km, 4km or 8.1km terrain sizes, you can choose any size you want!
When you change terrain sizes, you need to edit TERRAIN.xml config file, all the *_weight.png images and map01_dem.png image. Please note that *_weight.png images are normal 1024, 2048 etc resolutions while map01_dem.png is one pixel larger, like 1025, 2049 etc. However this GDN post says about some layer image, but I cant see one heh. Also this post, this post, this post, this post, this post about large terrains.
Change resolution for pda_map_H.dds image in MyTerrain\maps\ dir.
Change config of MyTerrain\maps\map01.xml file, on the line: ingameMap filename="maps/pda_map_H.png" so the width and height matches your new terrain size in meters.
After you've increased the PNG and DDS sizes, you must load the map01.i3d in Giants Editor FS17 and save it, not sure what it changes but this is required (otherwise FS17 hangs on load). You should save and exit your terrain in GE, only after that edit heightmap and _weight PNG images, do not edit those PNG images while GE is running, its recipe for a disaster.
And remember, you can do any size terrain you want, see Terrain Use Actual Size (FS19) tutorial for details.
General Size Editing Information
Giants FS17 Goldcrest Valley and FS17 Sosnovka terrains size of the map01_dem.png is 1025 x 1025 pixel resolution.
So for medium size terrain you need to scale that file to 2049 x 2049 and for large terrain to 4097 x 4097 pixel resolution. Just use 2048 x 2048 and 4096 x 4096, let Giants Editor FS17 add that extra pixel for you there.
To enlarge the terrain further you could increase the unitsPerPixel (FS19) attribute of the terrain (which has to be adjusted using a proper texteditor).
unitsPerPixel has value of two in FS17 Goldcrest Valley and FS17 Sosnovka terrains. Edit the terrain with a texteditor and set it to 3 or 4, save it and open the terrain in the GIANTS Editor. The world will be larger 