President Reagan decided yesterday on a range for the reductions in projected defense spending he will have to make to achieve his goal of a balanced budget in 1984, but will not announce his final decision until early next week.

White House deputy press secretary Larry Speakes said Reagan ended a 2 1/2-hour meeting on the defense budget by indicating the range of reductions he favors, and that Office of Management and Budget Director David A. Stockman already has a good enough idea of the president's thinking on defense to begin telling other departments and agencies what they must slash to keep the Reagan budget on target.

Senior administration officials have said that Reagan is willing to cut the 1983 and 1984 defense budgets by up to $30 billion. Speakes reiterated yesterday that such cuts would not prevent the president from keeping two promises: a balanced budget in 1984 and a 7 percent rate of growth in defense spending each year after allowing for inflation.

"There's no retreat from our determination to build up our military forces," Speakes quoted Reagan as saying.

The president also repeated his pledge to keep the fiscal 1982 budget deficit to $42.5 billion. In a brief question-and-answer session with reporters at the start of his defense budget meeting, Reagan acknowledged that some economists say the deficit will be much larger. He said pressure from high interest rates and the fact that he did not get everything he wanted from Congress make the task harder, but:

"Our own estimates show that it is manageable and that we can do it. And that we're going to do it."

Defense Secretary Caspar W. Weinberger, presumably for the last time, made clear his preference that there be no reduction in the March Reagan proposal for defense spending of $253 billion in fiscal 1983 and $288 billion in fiscal 1984. He spoke for 45 minutes, and also outlined various possible levels of budget cuts, Speakes said.

Stockman followed with a presentation of similar length, making the case for the levels of cuts he believes necessary to keep the Reagan budget process on track.

When a reporter remarked that Weinberger looked a little lonely seated on one side of the Cabinet Room table with a small gap on each side of his armchair while Reagan faced him across the table with Vice President Bush, Treasury Secretary Donald P. Regan, Stockman and others ranged in a tight row, Weinberger smiled and said quietly:

"They never promised me a rose garden."

For more than two weeks the administration has been drawing public attention to its new effort to reduce federal spending.

In part, the drawn-out operation, described by many officials' words and few numbers, is a public relations exercise designed to convince the financial markets that Reagan is serious about achieving the first balanced budget since 1969. If the budget were balanced, the government would not be a heavy borrower and interest rates would come down.

Speakes said that other presidents have panicked at about this time in the budget process. Reagan, he said, "could encourage the Federal Reserve Board to increase the money supply which would bring down interest rates." But, Speakes added, Reagan knows "we would pay the piper" and so he is not seeking such a short-term fix. Speakes said the president rejected any effort to influence the markets.

"We're a free-market operation. We think the best way to make the rates come down is to show that the program will work," he said.

Speakes said that Reagan believes the financial markets doubt that he will hold the line on his budget. "That's why he is determined to hold the line," Speakes said.