The merger of NationsBank Corp. and BankAmerica Corp. will give G. Patrick Phillips responsibility for the fifth-largest credit card business in the United States. But size is not the lone barometer of leadership, said NationsBank's president of financial products, who was designated president of the card services group for the new BankAmerica. "I would argue that we are already leaders," Mr. Phillips said in a telephone interview. NationsBank brings into the coast-to-coast megabank the second-largest debit card and 11th-largest credit card businesses, cobranding relationships including U S Airways, several smart card pilot experiences, and the second-largest commercial card program. (See article, page 12A.)