Would look like this in the console:
Warning, maps above 8k are just experimental ! They will take very long to generate, and might require higher memory allocation.
set prefablists to I:\7d2d\NitroGen_WorldGenerator/resources/prefablist.txt
creating output directory: NitroGenMap I:\7d2d\NitroGen_WorldGenerator\output\NitroGenMap
output directory created
running generation thread
... running terrain generation ...
image saved: I:\7d2d\NitroGen_WorldGenerator/output/NitroGenMap/radiation.png 102 ms
place: 57 flat , 24 rough, 37 desert, 26 mountains
exporting (1/3)
exporting (2/3)
image saved: I:\7d2d\NitroGen_WorldGenerator/output/NitroGenMap/genHM.png 6904 ms
exporting (3/3)
image saved: I:\7d2d\NitroGen_WorldGenerator/output/NitroGenMap/biomes.png 4191 ms
... terrain generation ... DONE ...
... running world mapping ...
gen custom settlements custom_town_size_15
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....
....
....
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exporting (1/3)
exporting (2/3)
I:\7d2d\NitroGen_WorldGenerator/output/NitroGenMap/biomes.png not TYPE_INT_RGB, copy type 6 -> 1
exporting (3/3)
... world mapping ... DONE ...
FINISHED GENERATING NEW WORLD
Total time taken: 3 min 41 sec
I had the same issue when I used Prefabs with the UNIQUE entry in the prefabslist.xml.
After removing it, every map size was possible to generate.