The width should be able to reach quite far, but the second field for height I'm not certain copes 400 000. Not even maps like Ascension island has that much, maybe it's a typo but even 40 000 in height is quite massive. Try something more reasonable like 1000 maybe even less.
Try to do smaller size increases on the width instead of just one huge leap, place a tree or something on each end and run through the map yourself so you can grasp just the mere size of it and then see if it's big enough for you.
It was a while ago since I did this but I think my game crashes of a 999 thousand width sized map. Game cannot really handle it well, I honestly do not know how the game would react just to a map in the 99k sizes with a heightmap on top of it either, neither do I see any use of having a map any bigger than 25 000-40 000 since the details in the heightmap just becomes big ditches or hills. Unless you just want empty space to run around on that is.
