The optimal size for all stacks ("wings") of all types of in-game aircraft units is five squadrons. Each additional squadron of that particular unit type that is added to the stack will add additional offensive and defensive strength but at less than 100% of the additional strength of the added unit. The concept is very much like the economics concept of diminishing marginal returns.
The optimal size varies significantly for other types of units from three battleships to eleven militia regiments. For convenience sake, I have copied and pasted a list below that was originally compiled by former administrator BRDubbs:
i. Aircraft – Interceptor – 5
ii. Aircraft – Naval Bomber - 5
iii. Aircraft – Strategic bomber – 5
iv. Aircraft – Tactical bomber - 5
v. Anti air – 8
vi. Anti air – SP – 6
vii. Anti tank – 8
viii. Artillery - 8
ix. Artillery – SP – 6
x. Armored car – 8
xi. Infantry – Commandos - 8
xii. Infantry – Infantry – 8
xiii. Infantry – Mechanized - 6
xiv. Infantry - militia - 11
xv. Infantry – motorized - 8
xvi. Naval - Battleships – 3
xvii. Naval – Carriers – 3
xviii. Naval – Cruiser – 5
xix. Naval – destroyer - 7
xx. Naval – submarines – 8
xxi. Tank - destroyer – 5
xxii. Tank – heavy – 5
xxiii. Tank – light - 6
xxiv. Tank – medium - 5
xxv. Aircraft - Rocket fighter - 5
xxvi. Railroadgun - 3
All credit for the foregoing list goes to BRDubbs; I'm merely re-posting his handiwork here.
