For the past 20 years, Eastman Kodak Company has displayed in New York's Grand Central Terminal the largest color transparency in the world. The Colorama, as it is called, measures 18 ft. high by 60 ft. long and is changed every three weeks.
George A. Massios,
"Underwater Photography On A Grand Scale", Proc. SPIE 0024, Underwater Photo-Optical Instrumentation Applications III, (1 June 1971); doi: 10.1117/12.953483; https://doi.org/10.1117/12.953483