The length of the line is the main consideration. Like reflex cabinets, you can 'cheat' with the bass response by having a port resonance slightly below the driver bass resonance in system. Meaning that the port is excited by output from the driver during its roll-off. Say for instance, the bass driver resonates at 40 Hz and rolls of below that, tuning the port/line a little way into this roll-off, say 35Hz, allows the two to sum and extend the bass output level to remain nearer flat to lower frequencies.

There are loads of books and websites that would explain better than me, take a look.