How can I get level's bounds (top, bottom, left, right, front, back)?

I need to do it at some point because of having custom level editor (for both the main game and user levels) which can be used to make levels of any size and shape.

Obviously I don't want player to fall forever if he gets out of bounds (whether due to a glitch or purposeful action of level designer), but also getting size of a level is important for AI reasons (while AI system I'm utilizing now is pretty barebones, in final game I want to use more advanced one that will require generating navmesh at runtime).