small vs. large port, same tuning frequency

I'll add to this as well.  Refer to the sticky subject on WinISD Pro:

• Input power as applicable.

"Use the 'Rear port air velocity' chart that I mentioned in the beginning to see what port noise looks like.

I have my chart shown set to feet/second and specified to show RMS, not peak, output.  This pic shows a port noise of 55 ft/sec, which is great for car audio because it is 5% of the speed of sound.  You should always keep it at 10% (110 ft/sec) or less.  The vent opening area will determine this noise, so making the vent smaller in diameter will make more noise, which might become unacceptable.  If you are working along with this guide, you will have to give your sub some power in order to get a port noise response.  Select the 'Signal' tab on your working box and input the sub's RMS, or the power that it will be getting, in that block labelled 'Input power'.  Now your port noise chart will jump to attention."

So to answer your question about best port diameter to use, the port air speed must be top priority.  Other factors that will influence port opening size are:

• Available space on the box for the port
• Room inside the box for the port length

For car audio sub boxes, the slot port is the easiest to design and build, and adds to the structural integrity of the box.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.