Yes you can get case fans bigger than 120mm, and they will move more air
at same speed. Not alot of places sell them though, and they are pretty
expensive. Also there comes a point when you don't want the whole side
of your case to be a fan....at least i feel that way, and alot of cases aren't
even wide enough to accomodate a fan that large. Sunon makes a beastly
176mm fan that pumps ~325 cfm, but at 65db it will probably sound like
a 747 at takeoff. 8O