Being & Building Followers & Friends of Jesus
I think that what is key to any small group or discipleship group, regardless of size, be it 3-4 or 12, is that it is centered around Scripture. Often it seems that groups centered on people and their prayer needs, as Jeremy H mentioned, groups diverge into social clubs. Scriptural fellowship seems to be best defined as mutual encouragement and edification centered around the Bible, the classic texts being Heb 10:24-25 and almost anywhere in 1 John [if I may be a little tongue-and-cheek]. However, we often overlook the obvious: we are gaining our definition of fellowship from the bible. We are called together in Christ by faith through the written testimony passed down to us from the Bible. The best thing Christians have to talk about is Who God is and what He has done for us in Christ according to the Scriptures (I Cor 15:3-4).
I confess, I'm a little off-topic, for the issue here is about size of a group for maximum impact. But I think the centrality of Scripture in a discipleship group is the first or second most important element of a discipleship group because by having the group thusly orientated, you keep the right Person as the focal point of the group, the Triune God Himself, and in turn, provide the ripest opportunity for another to engage the group: the Holy Spirit. At that point, I guess every discipleship group has to have at least five persons, three of them divine and at least two seeking the other three.