SUPERBANK is clearly at hand in the United States, heralded by the prospective merger of Chemical Bank and Manufacturers Hanover. The new bank will have combined assets of $135 billion. Citicorp, the nation's largest banking entity, has $217 billion in assets.Bigness is hardly a US phenomenon. US banks lag far behind overseas competitors in sheer size. Only one major US bank company, Citicorp, is found on the usual roster of the world's 10 largest banks; most are in Japan. Exchange rates have played a part in global rankings. Still, just a few decades ago most of the world's biggest banks were US, not overseas, institutions. Even while overseas banks have been growing in clout, many US banks have been hard hit by troubled real estate loans, as well as dubious loans to the third world. Operational costs have also escalated, as banks opened new offices, added expensive technology, and enlarged support staffs. So restructuring and mergers make sense. Indeed, more consolidations can be expected. One is already under consideration: NCNB Corporation and C&S/Sovran, two banks in the Southeast and South, who together would form an institution with $117 billion in assets. In response to changes in the banking field, Congress and the White House are seeking to draft comprehensive national reform legislation. Larger banks want interstate banking; industrial firms want to be able to own banks; and some bankers and brokerage house officials want to scuttle New Deal prohibitions barring investment houses from owning banks. Commercial banks need to be freed to compete more effectively against overseas rivals as well as the new "nonbank" banks found throughout the United States, such as American Express and Sears, Roebuck. On the other side of the equation, Congress and state legislatures should ensure that bank supervision remains certain and precise - to avoid the problems of the savings and loan industry. New Jersey is requiring that banks provide "no frills" accounts for poor people. The Garden State's plan warrants attention. Many depositors find that they can't afford to open a bank account, given expensive service charges - or that they don't qualify. Banks, understandably, do not want to be saddled with added "social welfare" costs. But in an age of giant "superbanks," banks shouldn't forget the individual. Indeed, many of today's most successful banks have sought to balance their services to meet the needs of consumers as well as commerce.