so I have done quite some ported doorpanels in the past. Majority of them were in range of 10-12 litres and diameter of port from 25-40 mm - lenght is determined by enclosure volume, target tunning and port diameter. I got best results when port opening faced to kick-panel area or to the cars floor. I recomend some kind of aeroport ending or at least small roundover at port mouth to suppress port noise. Put appropriate damping materials into the ported panel - foam. Also consider to put a few damping plates on the inner enclosure walls to prevent enclosure ringing and resonating.

Here is the link to one of my installs with ported midbass enclosures