NEW YORK -- Edward I. Koch, the only mayor in America with a foreign policy and opinions on everything from the Yankees to Daniel Ortega, has a new assignment: bringing peace to Nicaragua.

As befits the head of a small city-state, Koch will be accompanied by a 30-person news media entourage, with people from six local television stations, when he leaves for Managua today. He is heading a fact-finding team to monitor promises by the Sandinista government to expand freedoms.

The 62-year-old Democrat wasted no time in publicizing his new diplomatic role. Last week, he used his regular television show, "Koch on Call," to quiz Assistant Secretary of State Elliott Abrams on U.S. policy toward Central America.

Koch dominates the news here like no mayor in recent times, and the news media have a seemingly insatiable appetite for anything he says or does. He in turn is so concerned with the news media that on a recent trip to Poland, he had the New York newspapers flown in daily so he could see what reporters were writing about him.

Two weeks ago, when the city was shaken by the biggest stock market plunge in history, the mayor was asked at a news conference about its impact on the local economy. In typical fashion, he viewed the question in personal terms.

"I don't know what my own stock portfolio suffered," he said. "I'd be shocked if I didn't suffer the same 20 percent loss."

As for the market's health, Koch said, "There are no experts in this area . . . . If you listen to them, it's gobbledygook . . . . That proves the rule that I have -- 70 percent of what anyone tells you is BS. If they're experts, 30 percent."

Koch then launched into a rambling, 15-minute monologue that careened across a wide range of topics and castigated critics on a subject that no one had asked about.

Yet this "long song and dance," as Koch put it, still plays to good reviews after nearly 10 years on the mayoral stage. Whether the high-pitched voice is passing judgment on subway gunman Bernhard H. Goetz or Jesse L. Jackson, New York developer Donald Trump or Polish union leader Lech Walesa, someone will print or broadcast it.

For local reporters, Koch is almost too accessible, a man who can commit news at any hour. Beyond his daily barrage of news conferences, Koch pauses three or four times a day at the radiator in the lobby for impromptu chats with reporters.

"It's a nightmare for a reporter, because you have to cover him all the time," said Marcia Kramer of The Daily News. "I had to join a gym to work out in order to have the stamina to cover this guy."

"President Reagan's had three press conferences this year," New York Times reporter Alan Finder said. "The mayor has that many in a good morning."

Sometimes Koch even scoops himself. He disclosed the eight-day mission to Nicaragua to editors at New York Newsday days before he was scheduled to make the announcement in a speech in Michigan. "He loves to give out news," press secretary George Arzt said.

Koch was asked to visit Nicaragua by former Virginia governor Charles S. Robb and a private group in Washington, who perhaps were not unmindful of his built-in news media following. This is a mayor who lectured President Jimmy Carter on Israel during the 1980 presidential campaign and publicly interrogates foreign dignitaries during ceremonial visits at City Hall. Once, while reading a proclamation to a group of Soviet schoolchildren, Koch told them their government was "the pits."

The author of a best-selling autobiography, Koch juggles six newspaper columns, a television show and two radio shows, and he reviews movies for the New York Post.

Meanwhile, some friends and critics say they believe that the three-term mayor has become too reliant on snappy one-liners and less interested in the mundane details of governing. Koch insists this is not the case, but he became increasingly subdued in the wake of the city's corruption scandals.

After a minor stroke in August, however, Koch seemed to rebound on a wave of public sympathy. Once again, he was giving advice on dieting (he posted his daily weight loss until he gained back a few pounds) and health care (he put out a 28-page account of his hospital stay), and publicizing plans for his funeral (at a reform Jewish synagogue, with John Cardinal O'Connor delivering a eulogy stating that "he fiercely loved the people of the City of New York").

Koch's bald pate appears in numerous city ads, and it is often hard to separate the mayor's boosterism from his penchant for self-promotion. When a reporter asked what he viewed as New York's greatest advantage in its battle over business with New Jersey, the mayor replied: Ed Koch.

A man who derides critics as "crazies" and "wackos," he does not hesitate to take on journalists who challenge him.

On a recent WCBS-TV news show, Daily News columnist Ken Auletta asked Koch why he had invited a former top aide, Bess Myerson, to a Passover seder despite allegations of corruption against her. Koch shot back that Auletta should understand his compassionate act, "because you're half Jewish and half Catholic."

Auletta says Koch can "bully" some reporters. When Koch announced his candidacy for governor in 1982, Auletta said, the mayor publicly assailed him for writing a column disparaging his chances.

But like most journalists, Auletta likes Koch and marvels at his news media wizardry. "Koch understands that TV and radio, and to some extent newspapers, are slaves to the 30-second sound bite," he said. "He knows how to deliver that morsel."

At his news conference after the stock market's dive, Koch seemed more interested in deflecting criticism that the city had been greedy in auctioning off land at Columbus Circle for a huge skycraper. Koch singled out some umbrella-waving protesters who were dramatizing the shadows the building would cast on Central Park.

"I'm so fed up with the people who are out there yelling, the city is overbuilt and there's too much development . . . . I see all these people . . . The Times said 800, TV said 1,000 . . . . They don't know what they're doing in terms of preventing new jobs from being generated in this town."

Although the critics are merely advocating a scaled-down project, Koch fired this final salvo: "It is easy to give credence to the naysayers {who say} shut down the city, no growth . . . . You think {developers} want to be perceived as the ogres and the barbarians and the philistines? Not many people can go through that day after day. I'm one of those who can."