It would be an understatement to say that deciding on the size of a team is an exact science! It is certainly a topic where there are a lot of divergent views. At a basic level, large teams have always been considered unwieldy and ineffective in delivering results while small teams are perceived to be a lot more sure footed and better at delivering results. These are generalisations, but most experts will tell you that the ideal team size lies somewhere between 5 -10 individuals. Over the years, there has been a lot of curiosity and research on this issue and there are several interesting finds pertaining to various business situations. Some of these findings are outlined below:

• A study released in 2005 by Quantitative Software Management (QSM) on software projects showed that, assigning a large team on software projects does not necessarily result in any significant shortening of the project time. It can actually result in more defects!
  (By large teams they mean teams containing 20 or more individuals and small teams are those with 5 or less individuals).
• According to author Stephen Robbins, when teams have more than 10-12 people, the team finds constructive interaction difficult. Teamwork principles of mutual accountability and cohesiveness that are necessary to achieve high performance become difficult in large teams. His advice to managers is to keep the team size to under a dozen.
• Research on top management teams, by Katzenbach and Smith, shows that these teams are far more difficult to form. However, they fare better as real teams when they are small. As far as the success and performance of top management teams goes, it is the quality, capability and attitude of each member of the executive team that counts.
• According to management consultant Kal Bishop, who studies ‘creativity management’, if the team gets larger it is likely to foster an unfavourable phenomenon known as group think, where the team members tend towards consensus rather than exploration of diverse ideas.
• There are innumerable companies that conduct team building programs and games. Team building programs are usually designed to improve cohesion, trust, cooperation and collaborative work within teams. The ideal team size to achieve this objective seems to be five or six.
• With regard to global, virtual teams, team size is not a major issue once a company creates proper systems in terms of connectivity, information sharing and shared processes. However, if the team becomes too large, say thirty or more people, research shows that it becomes difficult to maintain the focus. Karen Lojeski has researched this topic and advises managers that it is prudent to divide such large virtual teams into sub groups. (Karen Lojeski is program director for the business and technology program at Stevens Institute of Technology)
• In Start-up businesses the initial team, comprising the founders / ownership team, is usually small. By and large, such teams seem to have no more than three people. A study done in Europe in 2004 examines the relationship between start-up team sizes and the effort necessary. The study shows that start-ups with three individuals put in more weekly hours of effort compared to start-ups where the team size is five individuals.
• In a cross functional project team, the size of the team is dictated by the functions that have to be represented. So the team size varies by task.

As you can see there is no one model or ideal as far as team size goes. But it is now well accepted that team size is certainly a factor in team performance.

An interesting point to note is the size of Start up teams. Start-ups usually begin to take shape when a few key people with a common vision and an entrepreneurial spirit, get together and start a business venture. Though we know that many Start-ups fail, there are also several inspiring examples of Start-up teams that have been successful in building a strong business. This fact has an interesting implication on team size in general. What this really means is that a small team size of less than five people, even as small as two to three individuals can be extremely effective on a task, as long as the team members have a strong commitment to the shared vision, determination to succeed and the willingness to stretch themselves and put in extra effort.

The above compilation broadly outlines various perspectives pertaining to team size. It give some direction on how to view team size in different business situations. However, the critical factor to keep in mind is the importance of justifying the presence of each and every member on the team. The key deciders should be the individual roles, the complexity of the task and the need for a certain number of people to execute the job effectively. Overall, research does seem to indicate that ultimately, small is the better way to go when forming a team.

References:
More people, More Bugs, http://www.projectsatwork.com/content/articles/227526.cfm
Essentials of Organizational Behaviour, by Stephen P. Robbins
The Wisdom of Team – Creating the High Performance Organisation by Katzenbach and Smith
Creativity Management and Team Size, http://ezinearticles.com/?Creativity-Management-and-Team-Size&id=63030

Findings on High-Performing Teams, Stephen P. Robbins.