I have a 3 x 5 closet. I started with one 4" input fan, and quickly realized I needed two 6" fans at floor level. Go big. You need to draw air from outside the closet into the closet though a wall probably. Then you need an exhaust with a carbon filter if odor is a concern. Again go big. This should exhaust from inside the grow room to another room, attic, etc. Outside is okay if you don't have to be paranoid. The more air, light, good food and daily attention you give them, the more they will reward you. Everything you need to know is somewhere on RIU.