'There is no one optimum size...'
Exactly, cams affect the need, carb size, etc., etc..........even dead stock engines but one bigger than the other like the DOHCs need changes, ergo, 750, 900, etc., minor mods made in them to change that. The 750 used different airbox rubbers from 900 that moved volume a bit as well as the carb entry shape and the 1100 added another intake duct to mod entry pressure.
I mentioned paper filter as they have a bit of restriction that makes the airbox volume seem more solid or set in place. You could rate the volume before the air filter but still inside box as a third tuning volume too but getting pretty extreme. The bleed rate across the filter affects the CV carb slide working too, some used a K&N stock replacement filter instead of pods and just dumping the OEM paper filter to use that cost power too. The K&N simply did not have enough restriction and the OEM fuel curve as supplied by those carbs wanted that.
Hey digger, have you picked up on the fact that the higher tuned fours nowadays are getting radical enough they are beginning to sound like 2 strokes at full power and rev? I listen to them here on the freeway next door and hear them all the time, I'd swear I'm listening to hot '60s 2 strokes now.
All that wave tuning if dead on can change the sound of the engine too, get in the power band and the engine note changes bigtime.