The size of your port has to do with port area. How much area depends on the net volume of the enclosure, as well as your response goals and if the port is a slot port or an aero port. For a slot port you'll want 12-16 square inches per cube, for an aero 9 - 12 square inches per cube. Less port area gives added control, with higher compression. More port area gives less compression and is for overall output. So goals come into play.