I'll try to address the issues one by one:
1) The result looks weird, because the St_ Oswald scenery is made to be mapped onto a sphere while FMSXYZ uses cylindrical mapping. I know for sure, because I made the mapping for this scenery initially to be used with "Aero Club Bad Oldesloe" (and the author "borrowed" it without asking...).
There is a mapping difference between cylinders and spheres that becomes ever bigger the further from the horizon you are. You'll find a lot of valuable info at wikipedia, in the mapping section. Look up "equirectangular projection" and "mercator projection" to find out why
2) Eight textures of 256px is by far too small. Try with 8 * 1024px, gives a horizontal resolution of 8k pixel, that's OK. To get everything matching, you need to create a seamless panorama first (e.g. with autostitch). This is not a trivial task, you'll need to consult some tutorials to get it correctly.
If the visual impression of the model size or some landscape object is wrong, then you see a mapping error of the pano. You can rely on the camera calculation of FMS, it's works correctly in 3D-scenes. If dimensions appear wrong, your pano is either stretched (mapped incorrectly) or it has a horizon problem (wrong height or tilted).
The size of the pano scene has nothing to do with this. When you are in the center of a sphere or cube, it will look the same no matter whether its 500m or 1000m in diameter. Double distance and double size equal out each other. The big size is just to make sure you don't hit it normally.
The whole collision object thing is a different matter alltogether. It's too early to bother until the rest is working properly.
Another pano scenery for you to study: "Modellflugverein Große Heide 73" or MFV73. I made the FMS scenery, the pano image already existed for Reflex. No collision points because there is simply nothing you could crash into. You'll need to convert all .jpgs to .bmp format.