well, from the specs on a SWR-1222D (type-r 12" DVC 2 Ohm sub) you only need .75 cu ft of interior volume with a 3" port dia. and 9.9" port length. But the question is, how much space do you have or are willing to use. Keep in mind you may want to compensate for port and driver displacement as well if you want to strickly go by the specs. The following is a website that may help you build the box you want...there are other websites and programs that offer similar info, its up to you to select which one you like the best for your application.
http://www.the12volt.com/caraudio/boxcalcs.asp Good luck...