Match The Right Sub(s) To the Space
Here is the method for matching the subs to the space:
8" - .7-1.0 cuft
10"- 1.2-1.5 cuft
12"- 1.7-2.5 cuft
15"- 3.0-4.5 cuft
The port formula for the above box sizes is as follows: 15-16 square inches of port area per cubic foot of box volume, make the port 13 inches deep. The port is tuned to the box volume, not the subs, and tunes to 45hz (about right for SPL & daily driving).
Once you determine how much space you have available, divide the sub(s) into that space. Don't cram the space, its better to error on the big side than the small side. A 10" sub will outperform a 12" sub if the space isn't enough.