map_preview.png resolution is 2048 x 1024.
pda_map.png resolution is what your terrain is, 2048, 4096, 8192. Or is it... 2048 seemed to work fine on larger terrains(?).
Preview and map images are once again NOT FLIPPED, hehe. Do them as is, no need to flip vertically anymore like in FS17.
Remember to delete the source PNG image next to the destination DDS which is what FS19 reads, if you leave map_preview.png or pda_map.png into your terrain directories, FS19 will give log warning about raw format image.