I recently read an article on CNET (Sorry, Facebook friends: Our brains can't keep up) about Dunbar's number playing out the same way online as it does offline.  I've blogged a bit about this in the context of other topics, but thought it would be worth mentioning again as it seems to come up quite a bit in the debate on proper MROC sizes. 

For those not familiar with Dunbar's number, it's basically a mental limit to the number of "relationships" that one can maintain at any given time.  The term "relationships" here is a bit fuzzy for some people, but generally means that you know everyone else in the group and have a sense of how people relate in that group.  For more, check out the Wikipedia entry on Dunbar's number.

What does this mean for MROCs?

To me, Dunbar's number provides a starting point for thinking of the size of the MROC you should build.  If you're interested in having members develop connections with each other, and in turn learning through listening carefully to those conversations, then you should consider a community that doesn't exceed 150 active participants.