What type size mesh to keep out rats/mice?
I am assuming that this will be for the crawl-space at the foundation. I have tried regular 1/4" mesh screen, and they just chew right through this. I would use the lath screen, also called carpenters cloth. It is a lot heavy'er, and more difficult to chew through. As far as digging under, you might want to use some pieces of rebar, and use wire to tie this into the bottom edge of the wire mesh. Bury the bottom edge down about 6-8", just in case some critter tries to tunnel under. The wire mesh will be galvanized, so even if it does rust, it should last for quite a while.