It has to do with the width/length/depth of the port. WinISD is a good program to figure it all up.
Right now I'm looking at a box tuned to 32Hz, volume of 4 cubic feet. Dimensions are: 15x15x39.43. Vent size is 2x14, vent length is 12.2". I'm not done fiddeling with it yet, but that's roughly what I'm looking at. It'll give me 2.57db gain at 33.6Hz, and cutoff of -3db at 28Hz.
Look into that program, read the help file. If you need any more help, just yell.
so hey man r those the dimensions u want for that box? sorry to bother u so much but i wont have internet access after tommorrow. so email me before tommorrow and tell me if u want it and if u do then the dimensions.
I'm still deciding. It's going to take me awhile to figure it all up. Need to ask some people questions, and see if I can find a sweet spot. I'll also need to go measure the car that the box will be residing in, and that's a bit hard, it's a six hour drive to get it... So it'll be awhile.