For 32 years, Robert Nathan has been in charge of his economic consulting firm, Robert R. Nathan Associates Inc., here in Washington, building the firm into one of the nation's largest economic consulting firms. Now, at age 69, Nathan is giving up the leadership of his company and turning over his post as president to the companys youngest vice president, John Beyer, 38.

"Twenty years ago the firm would have folded if I left," Nathan said in a recent interview. "Now I think the continuity is assured.

"It's a good transition. Most of the other vice presidents are very happy with John as president," Nathan said.

Besides giving up the management of his firm, Nathan in recent years has given up most of his stock in the firm, which he founded in 1946. The company is now owned mostly by about two dozen employes. Nathan retains a 20 percent, nonvoting share of the firm's stock.

Even at 69, though, Nathan is not contemplating retirement in the near future. Relieved of his management duties, he said he will not devote his full attention to consulting work. He also will remain as chairman of the board of directors.

Besides claiming that his firm is one of the biggest of its kind in the country, Nathan says it is also probably the oldest economic consulting firm in the nation. He said the firm had gross revenues of $5 million in 1977, up from about $4.5 million annually in the last several years.

"Rapid growth of the firm is not a major objective," Nathan said. "To maintain the quality of our work with rapid growth is very difficult."

Beyer, who took over as president June 9, has a background that is typical of the variety of experience that the firm values. Beyer joined Nathan Associates in 1966, then left the firm in 1968 to work for the Ford Foundation, first in Nepal and later in India. In 1973, he rejoined Nathan and in the meantime has worked on projects in numerous countries, including Sumatra, Guyana, El Salvador, Nicaragua, Malaysia, Colombia and Venezuela. He also has worked on several domestic projects, including economic studies of NASA's space shuttle and of a deepwater oil-transfer and refinery complex.

Nathan said his firm is unique among economic consulting firms in that all its work is "customized": the firm works only on specific projects for which it has been hired and does not provide any sport of general advice in a newsletter, for instance.

A large portion of the firm's work is done overseas, where about half of the 65 professionals are employed in 35 nations, mostly less-developed countries.