Before the limber figures, the picaresques, the colorized Aztec myths, before anything else, you’re confronted with the sheer scale of the towering fresco. Diego Rivera’s “The Marriage of the Artistic Expression of the North and of the South on the Continent” (1940) aka “Pan American Unity” stands 22 feet high and 74 feet long, and it’s unlike any narration you’ve seen before. Summarizing the mural in a neat paragraph is a fool’s errand