and the port size is up to you,it depends on what frequency you want the box tuned to. now port tunning and the cross over frequency that you tune the sub to are two seperate things,so don't get that part mixed up. the first thing you want to know is how many cubic feet of air space both subs will require and how many cubes you have in your car or wherever the box is going.it's at this point you can decide how you want to build your box.