I came across Dunbar's Number or the Rule of 150 while reading the The Tipping Point. I've noticed that the small companies where I've worked have been less than 150 people. BMH was approaching 150 employee's when it was purchased.
Since being bought out by a much larger company, the BMH operation has changed quite a bit. It no longer has the same feel even though I work with the same basic people. This probably has to do with the way in which the more common processes have been changed and the fact that one of the founders of BMH has become part time as he transitions into retirement.
Don't get me wrong about the above statements. I am not trying to be derogatory toward my current employer. It's more of comment/observation about the fact that small companies that want to grow need to realize that will need to change or they won't be successful.
Management has to prepare for the change and make that change easy or else they will lose the people that made them successful. (They may lose them anyway.) The Hutterite communities mentioned both in the article and the book recognized this early on and have a rule that communities must split once they reach 150 members.