For a given speaker in a given ported enclosure, a 2" diameter port will need to be roughly 5" long.
Now, for the same speaker in the same ported enclosure using a 3" diameter port you will need roughly a 12" long port.
For the same speaker in the same ported enclosure using a 4" diameter port you will need roughly a 24" long port.
So as the port diameter is wider; a longer length is needed.
Why?
Simple. It has to do with air pressure.
Now the difference between a slotted port and a PVC port is just physical looks and size. They both do the same exact thing. The PVC port will do the same thing except in a much smaller size.
Why go slotted then?
The bigger the port is in volume, the less chance of getting port noise. Port noise is extra noise coming from the port (sounds like wind).
Although a 2" diameter port by 5" long may be sufficient in air presure compliance, to tune your box at (? Hz), it will not guarantee that you will not get port noise.
So most guys just try to go as wide diameter as possible while not exceding port length that will be too long for their box. Obviously a 1 cubic foot ported box may not yield a 4" wide X 24" long port.