In "Taking Our Company to the Next Level," a lecture in the Entrepreneurship Through the Lens of Venture Capital series, Theresia Gouw is talking about organization structure in software engineering companies. Her thesis is: have an org structure, but keep decision making flat. There's a rule of 5 with direct reports, and a rule of 5 with teams.
I cracked up when, in the middle of this, she offers this caveat:
"...well, this is in software engineering. Obviously if you're building the next spacecraft and you've got significant complex systems it would be different."
Overall, her points do align with my personal experience:
"You get the most out of people when there are around 5 or 6 people on a team, in terms of productivity. And happiness, when you do the surveys. For what its worth."