Rosalynn Carter said today that President Carter is taking a "new approach" to Latin American foreign policy - "not to promise things he cannot deliver."

The first lady made it clear at a news conference in San Jose, Costa Rica, before flying here, that a major facet of the Carter administration's policy is to avoid rising unduly the expectations of other nations about assistance and trade adnantages from the United States.

U.S. officials traveling with Mrs. Carter said her comments were the first public enunciation of a new policy. They said President Carter signaled but did not articulate it in his April 14 speech to the Organization of American States.

In that speech, they added, the President made few commitments to Latin america because he realized that Congress is in no mood to increase foreign aid and because the government is pledged to focus its assistance on the poorest of the developing countries. Most Latin American countries are considered middle-income nations of the Third World.

Mrs. Carter is touring seven Latin American countries as the personal representative of her husband. After spending part of yesterday and today in Costa Rica, she arrived here with afternoon to begin discussions with the three-man military junta that has been in power since 1972.

Her comments on the President's desire to avoid rhetoric that raises false hopes came in response to a Costa Rican journalist who asked if she could persuade her husband to increase Costa Rica's trade with the United States.

She said she realized that Costa Rica wants to increase its beef sales to the United States but she said she told Costa Rican President Daniel Oduber that "Jimmy could not promise him anything that we could not deliver."

She added that U.S. Special Trade Negotiator Robert Strauss is reviewing thr quota system.

"We recognize and sympathize with the problems," she said. "We do understand the need for greater access to American markets for developing countries. But one of the policies of Jimmy's new approach is not to promise things he cannot deliver."

At another point she said of meat imports, "Those quotas are set. At this time we cannot promise the Costa Rican people that we can increase the market quota, but we are reviewing the whole situation."

U.S. officials here said Mrs. Carter was seeking to make a clear distinction between current policy andthat of the Ford administration.

In 1975, then Secretary of State Henry A. Kissinger promised a quota increase to Costa Rica "that he could not deliver," one official said. The official noted that in 1974, Kissinger pledged that the United States would have special relationships with Latin America. The new administration wants a "balanced relationship in which we deal with Latin American governments as individuals the same way we deal with the governments of Europe," the official said.

At the news conferece, Mrs. Carter was asked several questions about Robert L. Verso, the U.S. financier who has lived nearly five years in Costa Rica, avoiding prosecution on charges of fraud and obstructing justice in a securities case.

Mrs. Carter said she had not discussed Vesco with Costa Rican leaders, who have been embrassed by charges that Vesco companies largely financed Oduber's campaign in 1974.