I need ALOT of help with my snow scene.

I am making a snow scene for my games campaign, I am having issues with the terrain, fog, and snowfall.

I don’t really know how to use Unitys fog system, how would I make moderately dense fog?

Also I am baffled as how to make an 8x8km map(default terrain is 2x2km).

Also I can’t seem to rig my snow particles to work, I attached them to the camera so they followed the camera but it was very very laggy, Should I make it cover the entire map or really dense snow over camera?

Edit->Render Settigns has everything you need for setting up the fog. Enable it and play around with the color, mode and density.
As for snow particles, for testing purposes it shoud be sufficient to make the particle system follow the camera. Keep the max number of particles in the lower three digits and check if collision is enabled for the particles as this can impact performance, too. Also try switching the Simulation Space setting from local to world and vice versa.