If you are plumbing an application using 1/4" polyethylene tube for example, the appropriate filter port size would be 1/4" NPT.
That rule is not always valid, but is a reasonable guideline in the absence of any other information though it is possible in some cases, with a 1/4" NPT ported compressed air filter, you will have oversized the compressed air filter for that application, and it is possible that with the same 1/4" NPT ported filter you will have undersized the air filter for another. More on this farther along.
The flow characteristic of a given compressed air filter may change, depending on the Micron rating of the element inside it even though the housing and the port size of that air filter remains the same.
For example, a 1/2" NPT filter housing can have a variety of flow capacities depending on what size of filter element is installed inside.
Recognize too that the flow through the filter can be determined by the compressed air demand of the application, not necessarily the compressor size.
A 25 HP compressor can theoretically produce 100 CFM of flow at about 90 PSI, so with a 25 HP compressor it would be logical to think that you would need a filter with a capacity of 100 CFM to handle all of the air from that compressor.
But, what if you have a 200 gallon reservoir, and the downstream demand is for 150 CFM when a specific piece of equipment starts? The 25 HP compressor can produce 100 CFM at 90 PSI, the filter is sized for 100 CFM, yet if the receiver is big enough and the downstream air demand high enough, the flow of air to that application from the receiver might exceed the capacity of the filter for a short time.
It is important then to have a pretty good idea what the maximum plant CFM needs could possibly be if all demand was on stream through one compressed air filter at one time to have enough filter capacity at the receiver for that total demand if that should occur.
The reason is that even though the system will have an appropriately sized air filter at the receiver or compressor tank outlet, the nature of compressed air is that there will likely be condensation of water in the air lines after the initial filter. You will need to install a filter at each air consumption point to remove the water that has condensed in the air lines between the tank filter and the air using component.
If it is a filter that contains a general purpose element, and the NPT port size is the same inch size of the air line, usually you will be OK with using that filter in a typical air line / air tool application.
If in doubt, locate the vendor / manufacturer / model number information on the filter head or housing, and contact the vendor for flow ratings for that unit.
After you have dumped the compressed air from the line, and removed the filter bowl, by describing the element to the vendor or distributor, they should be able to identify the type of element and the flow rating of that compressed air filter with that element installed.