Terrain Editing Tutorial: Terrain User Interface Image Resolutions for Farming Simulator 17 (FS17) by PMC.
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map01_preview.png resolution is 2048 x 1024 pixels.
pda_map_H.png resolution is 2048 x 2048 for small terrains, 4096 x 4096 for medium/large terrains. It really depends what you can live with image pixel resolution quality wise, lower res and more low quality blur it looks like.
map01_preview.png and pda_map_H.png need to be flipped vertically otherwise they are upside down in-game.