the "Forbidden Area" is quite simple to set up, all you need is an "Area Shape" and a "Forbidden Area" Entity.
All you need to do is to draw an AreaShape around your map, and link the Entity with it like you would do with a sound (select the area and pick entity as target). I'm not intirely sure if the inside or outside of the shape is considered "out-of-bounds", but you can check that very easily by jumping ingame in the editor. In case it's the "wrong way" by default, there is an invert-option for the "Forbidden Area" entity.
Be aware though, the "Forbidden Area" entity cannot be selected by clicking on it for some reason (same with the factory entity btw), so in order to select it and change it's properties, place it on a separate layer and use the select object panel (Ctrl+t) to find and select it.
As for the minimap;
In the environmenttab of the rollupbar there is a tool for minimap creation...
You usually want the resolution to be 2048. After that, you'll need to add the forbiddenarea to it in photoshop, and then save it as a dds-file
(DXT1 RGB 4 bpp | no alpha no mips), actually you should be using the Crytif-plugin for photoshop, but I couldn't get that working properly (bugged piece of crap) so for minimaps and loadingscreen you can use the nvidia-plugin as well. You could take anything you like just make sure it's the same kind of dds as the minimap, except a resolution of 1024x1024 will be sufficient for that one.