I am definately no expert but I believe most SPLer's tune the box to the car's resonant frequency. This is the frequency when played in the car will exibit a gain in SPL. The only way to find out your cars resonant frequency is with a meter. Once you have this frequency you tune your box a few hertz below and then burp at the resonant frequency.

I think I read on the DD site that you should have at least 16" of vent area for each cubic foot of volume.Also bigger is not always better for internal volumes.

I am sure there are more educated members that will correct any incorrect assumptions I have made...

Best of Luck!