Use 12.5″^2 of port area per cubic foot of internal volume in the enclosure.  This is about ideal for most daily installations. Surface area is the width of the port multiplied by the height of the port.
Example – We would use 25″^2 of surface area for a 2.0ft^3 enclosure.

With 12.5″^2 of port area per cubic foot, you can use the following lengths to get one of the desired tuning frequencies.
16″ = 40hz
20″ = 36hz
25″ = 33hz
32″ = 30hz

With any of our subwoofers, we recommend that you tune no higher than 38 hz for daily music use.