Bane5 was discussing wing sizes specifically for the CAS aircraft type. The number of CAS planes that can join a land battle is a function of the combat width of that land battle. If you assuming a single 20-width division is fighting another 20-width division, then the most CAS you can add to that battle is 60. Having a wing size of, say, 50, means you're short 10 planes that you could have used, had your wing only been bigger. A wing size of 100 means you have 40 that can't get used in the battle (unless you're taking horrible air casualties). The full width of a crowded province (ignoring changes due to battle tactics) is 80. Attacking from multiple provinces increases that width (40 per extra province), so 3x the maximum combat width in play can be quite large.
As for air-air combat, there's a lot of old advice about wing sizes that's been rendered out of date by the newest patches. Wing size used to have a fairly strong effect on the combat results, due to the way the mechanics were calculated. That's much less pronounced these days. So, a lot of people simply use wing sizes that are convenient for filling airbases, which are all sized in multiples of 100. Hence, wing sizes of 50 or 100. Some that find that too much micromanagement, and use larger wing sizes, perhaps 400. (Personally, I'm in the 50 or 100 camp. 50s tend to be earlier on, or types that I don't have so many of, like NAV. I don't find it particularly tedious to relocate air wings, and it's nice to have the granularity to give the assignments more flexibility.)
In the data files, there are some references to the maximum number of wings that can engage in air combat, either 6 or 8. So with a really large number of planes concentrated in one air zone, you might need to use larger wing sizes. This limit and its actual effects have been much debated on the forum. There's also a 2v1 mechanic in air combat. It's a good thing to have two planes gang up on one enemy, but it also seems that that limits the number of planes you can effectively employ to twice the number of enemy planes.
I haven't seen a lot recently about fine-tuning wing sizes to try to game the calculations to gain an advantage simply from the wing size. But then, I also haven't been paying close attention to the forum, so it's possible I've simply missed recent research and reverse-engineering.
Some basic background, albeit sometimes dated:
http://www.hoi4wiki.com/Air_warfare
https://forum.paradoxplaza.com/foru...-mechanics-combat-mission-quick-guide.947290/
