It's not so much the size of the vent as it is the cfm of the fan hooked to that vent.

Venting requirements also depend on the specifics of your particular setup - so a lot more information is necessary if you want an educated answer to your question.

Ceiling height in room (need cu.ft. of room) - lights - are lights cooled - ambient temps (vent reqs will be different in Siberia vs. Sudan ). Anyway, you get the idea.

PC :smokin: