If you have a 265 or larger fan,Whatever size your exhaust opening is.Then that should be the size of the intake.It will be a bit of negative pressure. But moderate. It's the same principle as your central heating and air system in your home.You can't have more cfm's than you are pulling on the return. If you were to cut a 6 inch intake and a 4 inch exhaust,you would cool the room and exchange the air at an even rate.A 6 inch hole is 100 cfm.Distribute the cfm accordingly.Like part of it hook to the light.A 4 inch hole is 50 cfm.

Ga Grown!