youll want the intake holes as low as possible as thats where the cooler air is in the room. heat rises cool air settles. just my $.02 if you have a fan hooked up like this...filter duct lights duct fan duct out of box, you create negative pressure inside the box. now with negative pressure you suck fresh air in thru the intake holes and exhaust out the other end. if you have a fan that has a high enough cfm this will happen. with the size of your box i would go minimum 6" 400 cfm. now if you go this route the intake holes should be double of the exhaust..meaning if you have a 6" exhaust your intake should be atleast 12"(2-6" round holes, 3-4" round holes etc) make sense? also think about putting a filter of some sort on them intake holes as you dont want any unwanted dirt, bugs going into your room. ( i dont use anything fancy ) i found some carbon activated filter (kinda like furnace filters but not quite at home depot) and used that over my intake holes. ive seen some people say they use hepa filters in theirs. to each their own, but definitely would look into that. im guessing a cotton prefilter would probably work just as good but i dont know exactly. i put a pic of my grow box in my journal if you wanna check that out to get an idea. the pic is old and doesnt have the updated ductwork on the one side as i was still in the process of finalizing the wiring but the concept is there. they are both 6" round holes too.