i beleive 60 cm is about 21 inches. that being said, how much cfm do i need?Im from the states so i have no idea what your dimensions represent in inches or feet,i do know that computer fans suck ass,dont waste your time with those lil fartin ass things,hit up a hardware store or a home depot & buy a couple inline fans,one for exhaust & one for intake.
You can never have too much ventilation,too much is still never enough,get as much as you can .