I read a most interesting comment on the proper sizing of scrum teams, I believe it came from Werner Vogel, CTO at Amazon:
If they can eat more than two pizzas, it’s too large
Eager to find out what other people think is an appropriate team size, I stumbled across this comment on InfoQ – http://www.infoq.com/news/2007/07/agile_team_size. This certainly matches my own opinion on this area. I’ve been working in agile teams for around a year now, both as scrum master and as a regular team member and I truly believe that the size of the team matters a great deal when it comes to the productivity of the team.
If your team is too small you tend to have too little capacity in your team. A team will only be really efficient if the members within the team have all the required skills required to solve the user stories /use cases at hand. Another reason for not having less than 5 members is that there is always some administrative overhead for a scrum team.
If you on the other hand have a large team, you will not have any problem with capacity in your team. However, plenty of studies have shown that large teams result in significant lower productivity. In a recent project we were 6 members in the team and had a velocity of around 25 story points per sprint. Now the team has started development on phase 2 of the project, the only difference being that the team size has increased, now counting 12 members. You would expect that the team with this size could deliver around 40-50 story points (if story point estimation is approximately the same), but this was not the case. Instead the team delivered about 30 points. I think there
So why does large teams have a lower productivity than medium sized teams? First of all, when your team has become a crowd there is an increased chance of miscommunication between the team members. Second, it is a lot easier to hide in a large crowd. So if you have members in your team who aren’t totally committed to their work, then you might have a couple of extra challenges at hand. Thirdly, in large teams there tend to be less focus then in a smaller team. A large team is obviously expected to deliver more functionality within an iteration which of course leads to more tasks for the team to solve. This will actually decrease the focus of the team as the effort is more spread. When you can’t fit all the tasks on a 1,5×4 meters task board, then you know you will have problems focusing.
Conclusion: It is my opinion that the optimal size of an agile team is between 5 and 7 people. In a recent project (running for 6 weeks autumn 2007) I had the joy of participating in a “hyper productive” team consisting of one scrum team with 6 members. One of the really good things about this team was that the members had all the required skills necessary to solve the tasks at hand, so there was no need to consult external resources in order to solve our tasks. Also, I found that the size of the team led to a really good team spirit and great team dynamics. Problems were solved in short design sessions followed by an immediate action – usually doing pair programming. And we never managed to eat more than 2 pizzas…..