Credit unions, which began early in the century to help the small customer disdained by large commercial banks, are themselves growing bigger all the time. And their recently announced move into the credit card field will boost their growth even more dramatically.

A joint accouncement by Credit Union Card Services, Inc., VISA U.S.A., Inc., and the Credit Union National Association, Inc., a trade group, this week said credit unions nationwide may offer the VISA card to members soon at a 12 per cent annual interest rate. VISA, formerly Bank-Americard, is offered by commercial banks to customers at an 18 per cent rate in most states.

The lower interest rate is possible because Credit Union Card Services will provide data processing, education and marketing support, and liaison between regulatory bodies and VISA, a CUNA spokeswoman said.

A pilot program involving nine federal and 14 state credit unions, primarily in California, Michigan and Rhode Island, already has begun. Others are expected to join early next year, spokeswoman Ellen Lovell said.

She said VISA was selected over other bank credit cards in part because it would not require the agencies to sign up merchants to accept the card. That activity is prohibited by credit union regulations.

Credit unions have enjoyed substantial growth in recent years. Between 1971 and 1976 assets of the 22,500 federal and state credit unions more than doubled to $50 billion. Nationwide, they have 35 million members.

Last spring, landmark legislation was passed enabling credit unions to offer consumer banking and mortgage services once provided only by commercial banks and thrift institutions.

The credit card service and the trend to electronic funds transfer systems are both outgrowths of that legislation.

About 80 credit unions across the country now issue plastic transaction cars to their customers for identification and as a debit card with which to gain entry to automated teller systems.Thirty of the agencies have or share automated teller machines.

In the Washington area, several credit unions have joined together in a program called Minuteman. Automatic tellers are located downtown and in Arlington, Suitland, Hysttsville, Silver Spring, Langley Park and Gaitherburg. Cash withdrawals are possible 10 hours a day, seven days a week.

Last month, CUNA and Sears, Rocbuch and Co. entered into a third party payment arrangement. The nation's largest retailer will accept and clear share drafts electonically for CUNA customers. Credit union members will not be able to use their VISA cards at Sears, however, because the chain doesn't accept credit cards other than its own.