Hi Panzer,

You need to adjust the map borders, no need to touch polygons or textures.
Note in the below pic, I am in map making mode (F2 button) and at top right i have clicked on the clip tab (or the number 3 on yr keyboard). On the clip tab i have 'show map borders' ticked. I have selected the auto preset (in blue). You now see the map borders as shown.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2185898155

Now move the mouse over the lines until the cursor changes to green, click and drag the borders to where you want them as shown below. Once happy with their location, save the map and reload it, your playable map size is now reduced.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2185898183

The yellow line is where your border fog starts, the green line is connected to the yellow line and cannot be moved seperately. The green line is the heaviest fog whilst the yellow is the lightest fog point.
The blue line is the camera restriction line which means the camera can not go beyond this line, but you can still see the entities beyond that line when playing.
The purple line is the unit move restriction line, your units cannot move beyond the purple line when playing a game however reinforcements and air units can cross this line when entering the game.