My two and a half cents.
i use a 100 micron and been doin so for a long time, i have had 8 rotating 100 miron socks, 4 at a time in my set up, the media is no where near dirty enough to clean yet, and my water is quite clear.
filter wool is much coarser than 100micron but that polishes up the water nicely.
100 micron is plenty, i ried 50, but thta clogs very fast.
and the clarity reward ratio to cleaning ws not enough.
as for 1 micron, i believe that will clog rapidly and will require pressure to push water through? i mean, if it traps even parasites, heck - you cant even see that and your filter will clog pretty quickly.
i agree with david though, to say that 50 or 40 micron is too coarse, frankly i find that ridiculous, canister filters will never provide more than 50 micron if they are lucky - but that is very succesful in keeping aquariums.