“GIANTS: The Twin Towers & the Twentieth Century,” commemorates the original World Trade Center, viewing its creation in the context of the technological ambitions of the 1960s and the hundred-year evolution of New York’s skyline.On their completion in 1971 and 1973, the Twin Towers were both the tallest and the largest skyscrapers in the world. Innovative engineering carried the structures to 110 stories, at 1362 and 1368 feet, and floors of nearly an acre multiplied the space of each tower into more than 4 million square feet. The scale of these giants has been exceeded only once in history, by the contemporary Sears Tower in 1974. By comparison, the Freedom Tower, the largest building planned for Ground Zero, will contain only 2.6 million square feet, about two-thirds of their volume.