Depends on your port's height.Start off by multiplying your cubic feet by 14-16 to get your crossectional area aka port area. Then take that number divide it by your height and you see how wide the port should be.
If you dont have Winisd, just download it, its pretty simple to use.
If you dont want to do that then use this
Subwoofer Enclosure Calculators, Parallel, Series, Port Length and Volume go to the slot port calculator and enter in your dimensions and area