More importantly when it comes to size (its not the size of the boat....) a medium sized guild with lots of active players (Alts and Mains) will rank faster than a larger guild with the same ratio of active to less-active as we have. Reknown on the other hand is gained by them faster down to more members able to do RVR and so on. However I havent yet discovered why a regiment/guild needs to earn Reknown.
Do you have any source for the guild ranking (medium vs large) other than player speculation?
All I have been able to find - admittedly I have only been researching it since yesterday as part of my general getting up to speed on game mechanics, so am not confident that I know it all (yet) - suggests that practically everything that is said in various forums about how to rank fast is invented by players.

To summarize what we do know from when this came up and developers answered:
a)
http://www.warhammeralliance.com/forums/showthread.php?p=1746103#post1746103b)
http://www.warhammeralliance.com/forums/showthread.php?p=1750395#post1750395c)
http://www.warhammeralliance.com/forums/showthread.php?p=1750446#post1750446d)
http://www.warhammeralliance.com/forums/showthread.php?p=1751202#post1751202e)
http://www.warhammeralliance.com/forums/showthread.php?p=1765292#post1765292f)
http://www.warhammeralliance.com/forums/showthread.php?p=1765636#post1765636From a) any active character that gains XP contributes XP to the guild
From b) only active members are counted, inactive alts/mains do not count for either positive or negative aspects - it is the same as if they were not there at all, and that it is
supposed to work reasonably well no matter whether you are large or small, causal or hardcore.
From c) yes, a) also goes for those at 40 even if they do not, actually, gain XP due to being at the cap. They contribute what they would have were they not capped.
From d) what contributes active/inactive is up for guessing by the playerbase - i.e. no gaming of it for a long time, but likely to be many people thinking up pet theories ("surely, it is logical that it would be defined as...") and working as if their pet theory is the right one.
From e) yes, inactive accounts count as inactive for XP purposes, but the "are you active" formula has nothing to do with that

(oh, the horrible questions people ask)
From f) yes, if you play your main half the time and an alt the other half and are equally focused on doing stuff that'll gain you xp while doing so, you are either not at all, or only to an insignificant degree, decreasing your guild contribution compared to what it would be, if you were playing your main all the time.
How about a snapshot of WARDB:
http://www.wardb.com/search.aspx?sf=1&browse=10#guilds:0-6+1Yes, there are a lot of small guilds in the first positions, but overall large, medium, small... and gigantic (a friendly wave to the oversized Goon Squad

) are pretty much mixed up with no rhyme or reason at this point in time.
It
seems (i.e. I'm probably deluding myself

) remarkably likely that number of active players (the inactive have already been removed from the equation) means next to nothing compared to what those players, who are active, do while they are active - aka. a system based not so much on what you are doing at which level but on how much xp you get for the time you are online compared with how much one would be normal to get for the time (for some standard) scaled by average recent guild activity, probably interpolated with a spline or more complex structure, getting added to the guild xp.
In other words... While there are any number of good reasons for wanting a guild of a certain size or with a certain amount of activity for social and game goal related purposes, anybody who currently try to factor in "how will we gain guild ranks quickest" are, based on current knowledge I could find, probably fooling themselves rather than gaming the system (unless their "how to gain ranks quickest" is based on being very, very, active while they are active and hence accomplish more while actually online).