Robert Irwin, an environmental artist from Los Angeles, has been commissioned to produce a major sculpture for the atrium of the Old Post Office building on Pennsylvania Avenue -- the largest interior space in Washington.

The sculpture reportedly will cost between $65,000 and $85,000 although the final figure has not been determined.

The conceptual work will be located in the central courtyard or "cortile" of the building -- a space considerably larger than the atrium in the East Building of the National Gallery of Art, which is dominated by a Calder mobile. "This work will harmonize -- not dominate -- so as not to overwhelm the historic nature of this old building," said Robert Hammell of Arthur Cotton Moore Associates, the leading designers in the joint venture managing the building's renovation. "In a subtle fashion, it will heighten the experience of the space, emphasizing the verticality and immensity of it."

The work will consist of a 140-foot-long rectangular aluminum tube suspended 30 feet above the floor and parallel to it. Between the wires holding it up will hang 130 to 160 seven-by-nine-foot rectangles of translucent nylon scrim. The assemblage will mimic the pattern of openings in the walls of the cortile, and thus reinforce the experience of the space.

"Irwin studied the 'cortile' for several days and determined the dominiant visual element to be the 415 seven-by-nine-foot openings in the four sur- rounding walls," said Hammell. Oriented north and south, the structure will never block the view of the tower.

Though Irwin once stretched scrim across the bottom of the dome of the Corcoran Gallery's rotunda, he has not had a great deal of exposure in Washington. But Irwin, 51, is well known elsewhere. He had a solo exhibition at New York's Whitney Museum in 1977, and was commissioned to produce a work for the Olympics in Lake Placid. That work, Hammel said, "was too diaphanous to describe."

The General Services Administration recently undertook a $16-million repair and alteration project to transform the huge neo-romanesque post office building into offices, shops, restarurants and art galleries. The Irwin commission is part of GSA's Art-in-Architecture program, which allocates one-half of 1 percent of all new building and repair costs for works of art.

The artist was selected by a panel which included Jane Livingston, associated director of the Corcoran Gallery of Art; Hugh Davies of the University of Massachusetts; David Katsive of the Brooklyn Museum and Robert Hammel. By the present rules, all commissions are made by a four-person panel appointed by the National Endowment for the Arts. The panel recommended three artists, and the GSA chose Irwin. The rules are currently under review after the controversial placement of a $50,000 sculpture by another California artist, Guy Dill, in Huron, S.D., last summer.

The Irwin project is not expected to be completed for two years.