Commuting through a lot of rain recently with my Soma Es 64cm + 28c Marathon + tires. I'm kind of a newb at all this - and I assume (aside from wanting to stay dry) that the main purpose of fenders is to prevent all the gunk and dirt getting into components?

So looks like the best options are SKS Longboard and Planet Bike Cascadia? Can someone also advize on size I need? Does the 28c on a tire spec mean 28mm?