According to executive compensation data research firm Equilar, the number of directors on a company's board directly correlates with the size of the company.

Equilar's evaluation of 1,185 companies in the S&P 1500 found that the most common number of directors on a board is nine, while the average size of a board is slightly higher at 9.5 directors. It said that at the low end, there are two companies with only four directors while the company with the largest board had 34 directors.

“Larger companies typically have larger boards,” Equilar said in the 2011 Board Composition Series Report. The study revealed that the average size of a board among small-cap companies in the S&P 1500 averaged 8.4 directors last year, while the average size of a mid- and large-cap board consisted of 9.3 and 10.8 directors,  respectively.

While there is no prescription for the most effective number of members on a board, the report said over- and undersized boards can influence the decision-making process. According to the report, “A board that is too small may have trouble staffing key committees, and it may also require too much time for each director, influencing their ability to juggle board responsibilities with any day-to-day commitments."

When a company picks to have a small board, it can result in the lack of experience diversity and opinions to improve corporate strategy. If a large board size is picked, the problem will then be too many diverse ideas resulting in difficulties to reach an agreement or providing effective guidance for the chief executive officer. Other challenges can include inequality in the level of engagement from every director when the board size is too large.

The firm said over the past two decades, corporate boards of large U.S. companies have become smaller. “However, that trend has leveled off in recent years, with very little change in board size taking place in the last three years,” the report noted. Currently, boards have an average between eight and ten directors. In 2010, 53 percent of the companies observed had eight, nine, or ten directors.

Comparing board size across industries, Equilar found that most industries follow the convention of between eight and ten directors with a few exceptions. Technology companies prefer a smaller board (on average 8.4 members) than companies in other industries. They also found that companies in highly regulated industries tend to have larger boards as reflected among financial firms with an average of 10.7 directors.