It is a little hard to think of Ronald Reagan and Leonid Brezhnev in the same room, bargaining with each other about the limitation of nuclear arms. President Reagan has the ingratiating manner we all know, the grin, the disarming simplicity, the candid approach; Soviet leader Brezhnev has the heavyset face with the formidable eyebrows - he has been caricatured so often that it is hard to remember that he is human too, sometimes with a smile and even a gleam of humor. The two men have been in communication it seems, with private messages that are diplomatically polite, discussing the possibility of meeting. If the world is to be saved from ruinous and desperately dangerous confrontation they will have to hold a summit meeting some time or another.
Actually, they have met. Brezhnev, in his private letter of May 25 (responding to a personal note from President Rea-gan), recalls that back in June 1973 he saw Mr. Reagan when meeting Richard Nixon at Casa Pacifica. In their awkward relationship today Messrs. Brezhnev and Reagan seem glad to clutch at a little personal episode like this. ''Just as you do, I recall our brief conversation'' says Brezhnev pleasantly and continues, ''Today, as we did at that time, everyone in the Soviet leadership and I commit our hearts and minds to realizing the hopes and aspirations of all the world for peace, a quiet life and confidence in their future. . .''
Well, maybe. But certainly things were more hopeful in 1973. There were agreements that ''marked a radical turn for the better,'' the Soviet leader reminds the American President. There was a detente - ''peace and goodwill among men had never seemed closer at hand.''
The confrontation today could hardly be more complete. There is evidence of a kind of global scare. It is hard to estimate those peace demonstrations in Europe in any other terms. I am not talking about their logic or rationality. I would not try to argue with the warnings in this country either from the hawks or from the doves like George F. Kennan. The impressive thing at the moment is not argument but the simple, stunning size of the weapons involved. The seventh annual edition of ''World Military and Social Expenditures,'' edited by Ruth Leger Sivard, former chief of the economics division of the US Arms Control and Disarmament Agency, is just published here. It is a sort of handbook of where the world stands, with long matter-of-fact tables listing what some 150 nations are spending for soldiers, for teachers, for human resources, for safe drinking water, and for contrasting things like that.
Mostly the tables are put down without editorializing. Why elaborate? The facts are hard to face by themselves. The world spends an estimated $550 billion a year for arms, of which $100 billion is nuclear. Twenty-five million people serve in regular armed forces. Some 30 governments sell arms to other countries (the biggest buyers seem to be the poorest countries). Prices have gone up a lot. A World War II army tank cost about $50,000. Now the M-1, which was expected in 1974 to cost $900,000, is likely to run as high as $3,000,000. A lot of random figures can be culled from this compendium - ''. . . one Poseidon submarine carries 16 missiles, each with 10 warheads, and each warhead with over three times the explosive force of the Hiroshima bomb.'' In short, there are vastly more bombs today than cities to destroy and more coming all the time.
It is time Russia and the United States should begin talking face to face. Negotiations on one phase of limiting nuclear weapons have begun in Europe. The Reagan-Brezhnev letter exchange has begun. The President adopted a quieter tone in his foreign policy speech last week which got almost universal approval, though few think Moscow will accept the initial proposals.
''. . . We do not seek confrontation with the USA,'' wrote Brezhnev May 25 to the President, ''nor do we wish to infringe upon legitimate American interests.''
We needn't accept all of that. But it is certainly wonderful to have a change in tone for a while.