The research Hamster is referencing is Cowan[1] which follows from work by Miller[2]. Basically humans can keep track of a small number of 'objects' at once (objects sometimes including people in this context) before they start to abstract. The exact number depends on circumstances a varies from about 4 to about 9 depending on what is being kept track of. In a social game where you want people to be viewed as individuals it is prudent to keep the number of folks to be kept track of small, otherwise people with start treating each other as collectives rather than individual people, and that hasn't always worked out so well.
Since our context here is basically combat and psychology gives you variable (if interesting) results it pays to look at the anecdotal evidence of how similar teams are organized. I will consider military organizations like those of the US[3] or Britain, so this isn't universal, but it is illustrative. Roughly speaking the crew of a single ship in Guns of Icarus is equivalent to a fireteam. The typical number of soldiers in a fireteam is four, sometimes less. This is the second smallest unit of organization, the smallest being a support team, used when specialist roles are needed such as operating a mortar. I don't think it is a co-incidence that in Guns of Icarus we typically organize ourselves so that we have a captain at the top which generally instructs the gunners as to targets, below the captain is a main engineer with some command authority over a combat engineer. We have basically organized ourselves into a fireteam with an engineering support team.
Then again at the level of ship-to-ship interactions we encounter a similar effect. Groups of ships are a little like squads or sections. Typically these consist of between 7-12 soldiers, about two or three fireteams. I think this hints at something interesting. I find 2v2 and 3v3 matches are a lot easier to manage than 4v4 (the jump from coordinating 3 ships effectively to 4 seems very large to me), especially since I'm trying to manage my own ship at the same time. I suspect we wont ever see a 5v5 or 6v6 map even if the player base expands, not because of lag or performance (although those are probably also reasons), but because it would be very hard to command these things without further explicit or implicit command structures.
In the same way I suspect adding players would seriously disrupt this command structure and be very destabilizing. I also suspect it might make people less civil. As of right now many of us only get grumpy with folks if they ignore the communication side of things. Being crap or being new is okay, dumping an entire clip next to the target I ask you to shoot at is fine, firing it into the wrong target will not get you in my good book even if you hit every shot. If the number of players on a ship were changed we would abstract people into groups, and part of that abstraction is likely to conflate 'new and learning' with 'unresponsive', with undesirable results.
As a side note for those of you who captain a fair bunch the leaders of squads and fireteams are typically NCOs (non-commissioned officers). While it is tempting to think of oneself as a captain or admiral leading a ship or fleet, it is likely that given the game scale the field manual I referenced earlier or the works of someone like S.L.A. Marshall are more likely to be of utility than the works of Clausewitz, Jomini or even Sun Tzu.
[1] Cowan, Nelson (2001). "The magical number 4 in short-term memory: A reconsideration of mental storage capacity". Behavioral and Brain Sciences 24 (1): 87–114; discussion 114–85.
[2] Miller, G. A. (1956). "The magical number seven, plus or minus two: Some limits on our capacity for processing information". Psychological Review 63 (2): 81–97
[3] Department of the Army (1992-04-22). "Infantry Rifle Platoon and Squad". Field Manual Nº 7-8. Washington, DC, USA: globalsecurity.org. Retrieved 20 July 2011.
