President Reagan, in his State of the Union address tonight, will ask Treasury Secretary James A. Baker III to look into the problem of volatile international currency fluctuations and report back on possible ways to deal with it, administration sources said yesterday.

The proposal, which could lead to much broader action on international monetary reform, was described by informed officials yesterday as a "small, first step." One official said that it would demonstrate that the administration is "aware of the problem of volatile currency swings."

A second official said Baker would be urged to "go beyond" the emphasis in recent months on bringing down the international value of the dollar. The dollar has declined in value about 10 percent against other currencies since a decision in New York Sept. 22 by the Group of Five -- the United States, France, Great Britian, Japan and West Germany -- to act together to bring it down.

However, officials said Baker was not now being asked to take specific action on currency fluctuations but rather to "explore" what the administration might do.

The Treasury proposal in Reagan's address was being closely guarded by administration officials yesterday. It is among several ideas, some of them from earlier years, such as catastrophic health insurance for the aged, that have been added to the speech, to go along with the major issues of budget and tax policy.

The president is planning to visit the Treasury and Health and Human Services departments and speak to employes later this week to highlight these policy initiatives.

Baker has periodically mentioned possible U.S. action to stabilize currency swings and commented recently that he saw intensifying interest in improving the international monetary system.

The Reagan administration began with a free-market approach to the dollar, but the Group of Five meeting last year marked a shift in favor of currency intervention because the dollar was rising to new highs and making the U.S. trade deficit worse.

White House officials also said the estimated size of the fiscal 1987 deficit had been revised downward.

Officials said they now believe only about $38 billion in cuts is needed to achieve the $144 billion deficit target for next year. Previously, officials expected more than $50 billion in cuts.

The officials said the revision was due to technical factors, including the impact of $11.7 billion in automatic budget cuts required this year under the Gramm-Rudman-Hollings law.

The administration had come up with a list of more than $50 billion in savings, but officials said some are being put off until fiscal 1988, and others have been "repriced" so the total is only $38 billion. Reagan is to submit his fiscal 1987 budget to Congress next week.

In a speech to drum up support for the budget yesterday, Reagan said any further slowdown in his military buildup would "cripple our hope for successful arms talks with the Soviets, and we can't permit this." He added, "To cut the defense budget now any more than we have would put at risk the developing nations of the Third World, including growing democracies like El Salvador and Ecuador and would endanger the defense of Western Europe."

Presidential spokesman Larry Speakes announced that Reagan would, for the first time in his presidency, deliver a statement at the Government Printing Office when the fiscal 1987 budget is released Feb. 4.