uhh ok, dont know whats up with the other two guys but...
box tuning is determined by how big your box is, how big your port is and how long your port is.
(Im just making RANDOM numbers here for example) Now say I have a 2.5 cubic foot box and I want to tune it to 30 hz, and I want 15 sq inches of port per cubic foot; I would figure out how wide and how tall my port will be to get 15 sq inches of port per cube. Then I use a box calculator (like the one on the12volt.com) to figure out how long my port will need to be to make it 30hz. The longer the port, the lower the tuning.
If I forgot anything, let me know. Im half asleep right naw.