size all depends on the sub...
for music a lower tunning freq for ex...30 to 35hz range
and for all out spl the enclosure is usually tunned to the
resonance freq of the veicle..the last SPL box I built
ended up being tunned at 47hz

 

in Dave's car those same subs were getting about 1.2 cubes each and he was doing almost 152.

it all depends on the box design really.