President Reagan said yesterday in Brazil that the United States is discussing with its allies the possibility of expanding the multinational force in Lebanon, but Deputy Secretary of State Kenneth W. Dam told Congress that no decision has been made to increase the number of U.S. Marines in the force.

In a related move, the State Department said a congressional attempt to boost financial aid to Israel could have a "disastrous" effect on U.S. policy because it would take money from other deserving recipients and give the impression to Arabs and Israelis that the United States endorses Israel's continued occupation of southern Lebanon.

Dam also criticized Israel's insistence that negotiations on withdrawing its forces from Lebanon be held in Jerusalem. Pointing out that Lebanese acquiescence would jeopardize attempts of the new government there to obtain Arab support, Dam said, "Insistence on a Jerusalem venue should not be made an obstacle to the start of talks . . . ."

Testifying before the Senate Foreign Relations Committee, Dam denied a news report that the United States expects to double the 1,200-man Marine contingent in Beirut in response to a request by Lebanese President Amin Gemayel.

He said the administration has "neither accepted nor rejected" Gemayel's informal request for more troops. He conceded that "it may well be necessary to have an expansion of our force" to facilitate withdrawal of Israeli, Syrian and Palestine Liberation Organization forces from Lebanon, but he also cautioned:

" . . . We are reluctant to be drawn into a discussion of what the Marines might do in a situation that hasn't come to rest yet. It is premature to discuss the nature of an expanded force because it all depends on what the Lebanese, the Israelis, the Syrians and the PLO are able to agree on."

Reagan, on a Latin American tour, told reporters in Brasilia that the Lebanese "believe a bigger multinational force would help them in removing the other foreign forces" and that the United States is "talking to friends and allies" about what can be done. Later, Secretary of State George P. Shultz said no action would be taken without consulting Congress and the other governments contributing troops to the force.

The current 3,800-member force also includes 1,600 French troops and 1,000 from Italy. U.S. officials have said privately that the force almost certainly must be given more troops and wider duties as part of the effort to get foreign forces out of Lebanon.

But, as Dam reaffirmed yesterday, they also have stressed that no decisions can be made about the scope of an expanded force without a clear idea of what a withdrawal agreement will entail.

The State Department's comments on aid came after a Senate appropriations subcommittee voted Tuesday to add $475 million to the $2.4 billion Israeli-assistance package in the foreign aid bill before Congress. The full Appropriations Committee is expected to take up the $11.2 billion bill today.

Department spokesman Alan Romberg said the action would harm worldwide U.S. interests by taking money from other countries and would raise "the substantial risk of being misinterpreted by any or all of the various parties to the peace process."