The president of the Metropolitan Museum of Art in New York announced today that the museum's new $26 million wing for 20th-century art will be called the Lila Acheson Wallace wing in honor of the cofounder of Reader's Digest.

"She shared both her wealth and time with the museum," museum President William B. Macomber said, "but on a virtually unparalleled scale." Wallace, who died in 1984 at the age of 95, contributed more than $50 million during her long association with the museum. Her last gift included an endowment for permanently operating a major portion of the wing, which is scheduled to open in February 1987. The four-story, 110,000-square-foot structure is expected to cost about $2.5 million a year to operate. The Met's annual budget is currently $55 million.

New York Mayor Edward I. Koch, a featured speaker at the morning ceremony, committed city funds for a third of the new wing's operating costs. The city had already allocated $8 million for construction, which was completed Dec. 19.

"The Metropolitan Museum of Art," said Koch, "is a cornerstone of New York City's prestige as the cultural capital of the world. The museum will finally be able to display the work of living U.S. artists." The mayor singled out his first commissioner of cultural affairs, Henry Geldzahler, who served as the first head of the museum's department of 20th-century art, for purchasing key works of contemporary American artists.

"I don't know of another institution in the city," Koch said, "that so many people have such high regard for. I can hardly wait to come back when the paintings are on the wall."

Ceiling heights in the still empty galleries vary from 11 to 30 feet. Mezzanine and second-floor coves feature splendid views of Central Park and part of the city's fabled skyline. Giants of 20th-century art from Grant Wood to Pablo Picasso will be displayed from holdings that include 1,178 paintings, 373 sculptures, 3,104 works on paper and an architectural and design collection of 3,564 objects. Only the recently expanded Museum of Modern Art has greater exhibition space for 20th-century art. The architects of the Indian limestone-faced wing are Kevin Roche John Dinkeloo and Associates. Since 1967 the firm has designed all of the wings, pavilions and sculpture courts of the Met's master plan.