could close a port if not getting negative pressure. generally a passive port should be 2-3x larger area than the exhaust duct. i assumem stink is an issue for your locale, but if not, screw it.. it shoudln't stink much except for late flower... to my nose anyway... not overpowering anyway... nothing like harvest, holy shit...
definitely best to buy larger than needed and run at 50% or less power. gives room for error and will more times than not be quieter and longer life for the duct fan. "EC" motor with 0-100% control or 10-100% is what you want. those cheapies that come with a peniometer usualyl only give you 75-100% and their bearings suck ass. 60-70 for a 4inch 100ish for a 6inch with a couple good companies out there but i won't mention one of them... Terrabloom is the other. i have a bunch of them, the oldest 6" is 3 years old and still works like new. slimmer designs than the other "shithole" brand that is not to be mentioned by name, too.