The president's advisory commission on Social Security unanimously agreed yesterday that the giant system requires between $150 billion and $200 billion in tax increases or benefit cuts in the next seven years to stave off bankruptcy.

The commission, in the first day of a scheduled three-day meeting here, did not get to the question of how so much money might be raised.

But economist Alan Greenspan, the commission chairman, said he regarded agreement simply on the dimensions of the problem as significant because there has been so much dispute even over that. In general, Republicans have been saying major reforms are needed to save the system, and Democrats say that lesser steps might do.

One commission member, former U.S. representative Joe D. Waggonner (D-La.), said he thinks the commission and Congress will ultimately find the money, in part by curtailing cost-of-living increases in benefits and bringing federal, state and local government employes into the system.

Waggonner also said it seemed likely there would be attempts to move forward to 1984 tax increases now scheduled for 1985, 1986 and 1990.

In addition to their vote on the size of the near-term Social Security deficit, the commission members yesterday:

* Agreed on the dimensions of the system's likely long-term deficit, saying it would probably come to 1.8 percent of wages subject to the Social Security tax. This means that, without benefit cuts, a tax increase of 1.8 percentage points more than now contemplated would be needed to keep the system whole in the long run.

* Indicated they may recommend that Social Security no longer be counted as part of the federal budget, a step that assorted groups and experts have advocated as a way of partially depoliticizing the system. Though no vote was taken, eight of the 15 members spoke in favor of such separation.

The commission is supposed to report to the president by the end of the year. He will then make recommendations to Congress.

Greenspan and Robert Ball, a former Social Security commissioner who is one of five commission members appointed by House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) for the Democrats -- the other 10 were appointed by the president and Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) -- both declined to predict whether there would be a single majority report incorporating one or two items on which the entire commission could agree.

Several members said privately there may be two or three reports.

One would presumably incorporate the Democratic preference for raising new revenues without cutting benefits. A second would take a middle position, perhaps sponsored by Greenspan, that would raise new revenues but also curtail likely future benefit growth.

The possible third report would take a thoroughly "conservative" position and advocate that almost all the $150 billion to $200 billion be made up through benefit reduction.

Senate Finance Committee Chairman Robert J. Dole (R-Kan.), a commission member, hinted strongly Wednesday that he would be in the middle camp when the time comes for Congress to begin writing a Social Security bill.

Although he did not absolutely commit himself, Dole suggested that one acceptable solution would be to advance to 1984 and 1985 the tax increases scheduled to kick in over the next six years and to peg future cost-of-living increases to wages rather than prices, even in years when prices rise faster than pay.

Even though the election is over, there was some political sparring yesterday.

Pointing out that the $150 billion to $200 billion figure that the commission agreed on is based on rather pessimistic assumptions about the economy over the rest of the 1980s, Sen. Daniel Patrick Moynihan (D-N.Y.), a member of both the commission and Finance Committee, observed that, "We've just asserted the failure of Reaganomics."

Before the agreement on the target figure, Robert J. Myers, staff director of the commission, reviewed Social Security's financial position and told members that under any economic scenario the old-age fund could "safely get through 1983" by borrowing from the better-off Medicare or health insurance fund that the Social Security tax also supports.

But in 1984, he said, there would be nothing left to borrow and the old-age fund would face insolvency without further help.

In addition to setting the target figure, the commission agreed that there should be some fail-safe mechanism in case the sought-after $200 billion proved not enough. This would presumably be authority to borrow from the Treasury.

Besides the short-range shoring-up, the commission may recommend longer-range steps, one of which might be to phase in a normal retirement age higher than the current 65.

But one commission member who is also a member of Congress said that, at a time when unemployment is nearly 12 million, he saw no possibility of such a step because there are too many workers already.