The Silicon Valley "two-pizza" rule is often attributed to either Amazon's founder Jeff Bezos or its CTO Werner Vogels. It says that project teams should be limited to the amount of people that can be fed by two pizzas. 

Now, it's possible to take this rule too far - what size are the pizzas? How thick? What kind of cheese and toppings? If the meeting occurs just after lunch, two pizzas could probably feed tons of people. Just joking!

The spirit of the rule, though, is that the group should be small enough to be flexible and light; team members should be able to easily and freely improvise, innovate, and bounce ideas off of each other all the time. This leads to quick decision making, smaller hierarchy, faster response, more efficiency, and speed.