In his latest report George Barna talks about what people experience in churches, and finds not much difference in church size:
Many heated discussions occur about the optimal size for a church, but this data suggests that church experiences do not differ all that much based on the size of the church. For the most part, attenders of small, medium and larger churches described similar outcomes from their church engagement. Looking at moderate differences, attenders of mid-sized churches (defined as those with 100-299 adult attenders) were slightly less likely to report positive outcomes from church than were those attending larger and smaller congregations. Also, those attending larger churches (300+ attenders) were more likely than average to say they had gained new spiritual insight and understanding and that their church clearly prioritizes serving the poor.