The length of the port depends on what freq you want the box tuned at.
For more SQ try tuning lower.
28Hz would require the port to be about 20" long.
For more SPL try tuning higher
33Hz would require the port to be about 13.5" long
You could tune even higher for increased SPL. But you would really start to lose SQ. Your low end response would start to disappear with a high tuned box.
It all really just depends on what kind of sound your looking for.
Pioneer 8600 - Profile Amp-Infinity Speakers - L7 Subwoofer