This finding was a little odd -- most guilds in two games I looked at were small, very small. Many were either as small as possible or smaller than the smallest group size that the code allows. Specifically, the two games were both Sony games, Planetside 2 and EverQuest 2. Sony provides a census for both games, so you can scrape all the guilds for EQ2 but the API for PS2 is different so you can't for outfits (guilds) there, but there was a secondary website which has an unknown but large sample of outfits in PS2.
This is somewhat odd because we generally consider the point of a guild, at least from the point of the developers, is to allow and foster long-term group cohesion among many players so those players can go and do things that you can't do in solo play or small group play. However, a lot of players apparently don't actually use guilds in this way at all. They are using them, I think (at least in EQ2 which I play), for personal satisfaction in a variety of ways. (Personal satisfaction of leveling, design their own guild hall, "I have a guild", a "home" for the player's quiver of alts if not in a larger guild, a guild for their family members...)
Google doesn't have great table support, but I'll put some data here anyways.
For PS2, having an outfit of two players... well I'm not really sure what that does (in terms of the benefit).
PS2 (Capped at 21 here in this table but there are many larger outfits.)
|Outfit Size||How Many|
Here's a graph, capped at 50 on the X-axis.
There were similar results for EQ2, here shown by number of accounts. (PS2 and EQ2 do characters and accounts a little differently.) This, a census not a sample, is the subset of guilds which I determined were "active" during a 4-week period a few months ago when I scraped the census. There are far more guilds registered in EQ2 but most aren't active. Most guilds are small, but most accounts are in a large guild. A "group", as defined in code, well a full group, is 6 people, so guilds with 1-5 people can't fill a full group on their own (although with the recent additions of mercenaries they can, sort of).
Note that the "Accounts" column is non-linear. This is for the following reason: Full group = 6, x2 raid = 12, full raid (x4) = 24. Once it hits 24 I scale it.
|Accounts||No. of Guilds||Total Accts.|
And, a visual image of all that, I think each bar width is 5 characters. (Oops initially I put in the wrong chart, it had characters, not accounts...) Ok I am not sure why there is a discrepancy between the table and the chart with regard to the first two categories... that is.... slightly disturbing, but overall the point still holds.