The economic development of any nation is largely dependent on the stability of the nation’s banking industry, because the industry promotes the savings culture of the public, aids capital funding, implements the monetary policy of a country and promotes the facilitation of international trade. Therefore this study examined the effect of bank size on the financial performance of listed Deposit Money Banks in Nigeria (DMBN). Also, the study examined the moderating effect of internal control adequacy on the relationship between bank size and financial performance. The study adopted an ex-post facto research design because secondary data was extracted from published annual reports of 10 DMB in Nigeria, for a period of 12 years from 2006 to 2017. The data were analyzed using descriptive and inferential statistics (multiple regression analysis). Results from the analysis showed that the proxies of bank size (total assets, number of employees and customers’ deposit) had a cumulative effect on return on asset the proxy for financial performance. Also, internal control adequacy as a moderating variable enhanced the effect of bank size on financial performance. Hence, the study concluded that, total asset, number of employees ad customer’s deposit as proxies for bank size, had a combined effect on financial performance.

Keywords: Bank size, financial performance, DMBN, Internal control