"One of the most consistent empirical relationships about boards of directors is that board size is negatively related to firm performance (Hermalin and Weisbach, 2003). Yermack (1996) finds a statistically significant negative relationship between board size and firm performance as measured by Tobin’s Q with a sample of 452 large U.S. industrial corporations for the period of 1984-1991. In the same study Yermack also exhibited that companies with small boards have more favorable values for financial ratios. Similarly Eisenberg, Sundgren and Wells (1998) concluded the negative relationship between firm board size and performance measured by return on assets (ROA) for a sample of 879 small private firms in Finland. In their 1998 study, Barhart and Rosenstein revealed that companies with fewer board members have superior performance compared to companies with crowded boards (...)"
https://www.researchgate.net/publication/321869618_The_Effec...