Rain (and other enviornmental effects) are displayed through doodads (e.g. char ash). Scaling these dodads will manipulate the size of the weather effect. In regards to fog, I believe that it is set through triggers? I'm not entirely sure, to be honest.
Also- does anyone have any idea how to simulate utter darkness? There's the region specific night lighting options, (e.g. Mar Sara Campaign Night) but it is not dark enough.