The Semco story is fascinating. I wish I knew some folks that worked there. The wiki (http://en.wikipedia.org/wiki/Ricardo_Semler) doesn't tell much but from what I remember of the book, they cut organizations free when they reach a certain size.
I used to work for Viant, a digital consultancy where they kept every office to about 100 (or was it 150?) employees. If they had to hire more employees, they'd spin off a new office. This was based on Dunbar's number (referenced elsewhere in this thread).