I put it in on Malajuri, off the coast of a small island. The map looks quite tropical, so I think this is the best fit.

Now in the atlas:

If you have the crop image to aspect ratio option on when exporting, you shouldn't get those white bars on the top/bottom.

It gets a bit more interesting when using the dungeon walls, since a regular floor mask would cover up all the unevenness of the stonework. But I am wondering if you could use TRACED to generate a polygon made up from the total area of both the floors and walls here, perhaps with a distance of -0.5 or something like that to make the mask "hole" for the dungeon to give that below ground feeling. Probably need to do something with the path to the entrance too though, to make it look like it is sloping downward towards the entrance.

There is a bug with a few of the Cos3 templates. You can grab an updated version of the installer from your reg page, or download the fixed templates only from here. That should hopefully fix things for you (note that fixed templates means it will work properly on new maps crated from these. For existing maps, I recommend just copying things over to a new map once you have confirmed this fix works for you)

Your map is 250 by 200 feet, so that is 50 by 40 squares. Each square in FG is 50 pixels by default, so should be 2500 by 2000 pixel export if my calculations are right