This paper examines the impact of corporate board size on firm performance for a sample of 137 listed firms in Ghana and Nigeria. Our findings suggest a statistically significant and positive relationship between board size and firm performance, implying that in Ghana and Nigeria allowing corporate board size to be dependent of firm size tends to improve firm performance. Our findings are consistent across different kinds of models that deal with different types of endogeneities and corporate performance proxies. Our results provide empirical support for agency theory, which suggests that optimal corporate board size effectively advise, monitor and discipline management thereby improving firm performance.