The size of the port doesn't have much to do with the eventual sound you'll end up with. It's the length of the path the sound will travel within the cabinet, and then go out of the port, that will determine the effect created by the port.
Generally, I think the length of the path which the sound follows in the cab, to eventually exit through the port should be the same as the wavelength of the frequency(plus it's harmonics) you want to emphasize. You can influence the path length the sound has to travel within the cab by adjusting the cab's depth and/or building kind of a labyrinth inside.

eg: you want a boost at 300Hz

wavelength = 340m/s (speed of sound) / 300Hz = 1,1333m

Now build the cab with a depth of about: 1,1333 / 2 = 0,66m

Acoustics nerds, feel free to correct me. ;)