I did an R&D for NARAM-61 that looked at port sizing indirectly, as I was trying to show that some venting is necessary for good apogee readings and that if you don't you're cheating yourself of some "credit" for altitude attained. This all descended from, I believe, an awkward translation from Polish in the Adrel instructions that uses the term "hermetic" with respect to altimeter bays in competition-type model rockets. This in turn led some to say that there is no need to actually put vents in at all and two prior years' worth of R&D reports using bench testing to prove that and then show that there was nothing special about Adrel units in this respect. I just did actual flight testing version of the same experiment flying Adrels, FireFlys and MicroPeaks in multiple compartments on the same flight of the same model.
Of course some path for the pressure to equalize at apogee is needed or the pressure in the compartment never catches up to the outside air and you cheat yourself out of some meters on your altitude score. (See my graph in Dan Wolf's altimeter article in the Jan/Feb 2019
Sport Rocketry). But I found it was almost impossible, using traditional materials, to not get a reading at all (no launch detection), and that having enough pressure leakage to get good altitude readings required only the slightest venting, really—which corroborates Russ' first two sentences in post #9. Bigger vents (in this case 3 3/32 inch vent holes in a 2 1/2 inch section of BT-50 as the altimeter bay) made no real difference.
But when higher-than-model rocket speeds and bigger, heavier rockets that are depending on the electronics not only to report the apogee and perhaps give enough data to show altitude vs. time but to deploy the recovery system reliably, things need to be a little more carefully done. This leads to the report Cris linked to and Steve's comments just above.
This, of course, is way beyond what the OP was looking for to see if he was going to get reasonable readings from his FireFly