Somewhere around the blogs etc documenting Stackoverflow beta, someone asked "what's a good community size," and someone (Jeff?) answered along the lines of "we found it really started to work when it went north of 1000 people." I can't find the attribution right now... what's the link?
It was in reference to podcast 18. They were discussing the early Stack Overflow beta. At that point, 1,000 people had already answered a question with at least one up-vote (Teacher badge). The tone of the conversation was that Stack Overflow was declared "a success." I brought it up as a data point to consider. Not necessarily a minimum size for a "good community."
In more recent podcasts (episode 70), they talk about the success of StackExchange sites and the minimum number of knowledgeable people needed to make a site worthwhile. Joel's response: The minimum number of users needed answering questions: one.