WASHINGTON, DC.- The most lyrical of Christo and Jeanne-Claude’s epic projects was the “Running Fence, Sonoma and Marin Counties, California, 1972-76.” The ambitious scope and enormous size of this monumental temporary artwork are hard to imagine even today. The exhibition “Christo and Jeanne-Claude: Remembering the ‘Running Fence,’” on view at the Smithsonian American Art Museum April 2 through Sept. 26, captures the elaborate process of planning the work and the magnitude of its scale. It is organized by George Gurney, the museum’s deputy chief curator. “Running Fence,” the culmination of 42 months of collaborative efforts, was 24 1/2 miles long and 18 feet high, with one end dropping into the Pacific Ocean. The artwork was made of 240,000 square yards of heavy, woven, white nylon fabric, 90 miles of steel cable, 2,050 steel poles, 350,000 hooks and 14,000 earth anchors. Pa