intake exhaust ventilation

Im building a grow box thats 1'10" deep, 3' wide, and 4' tall...i have a 6 inch fan 230cfm for exhaust that im going to cut a hole for and connect to a carbon filter on top of the box, and im wondering how big my intake hole should be for a passive intake system. I plan on putting this hole in a lower corner of the box.

Any help would be greatly appreciated....