I was involved in a discussion recently concerning the size at which churches should split and plant a sister. We reached a consensus that, under normal circumstances, 300 is about right, but everyone agreed that it was somewhat difficult to explain why.
I don't have the stats on hand, but there are studies (studies!) that support such a position. I think the reasoning runs something like: The "connectedness" one feels toward a community is directly tied to the percentage of the community one can maintain a relationship with on an X level, and this percentage falls considerably when the community reaches 200 to 300 people.
I'm shooting from the hip here, but something else that may be apropos is that church-growth-manual type books all predict that small churches will hit a "ceiling" at the ~250 mark, a barrier that many churches are never able to ford. The books generally go on and talk about how a church should overcome such a ceiling, but perhaps we should be more attentive to the fact of the ceiling itself.
I also think churches should be more strongly tied to their community, and so geography should be a determining factor when considering and organizing a split, but I haven't any other thoughts on the subject.
Therefore, let me put the question to the reader: When should a church split and why?