Both sprawling and sparse, momentous and restrained, David Zwirner’s marquee summer show uses one room to display one work by six artists: Dan Flavin, On Kawara, Sol Lewitt, John McCracken, Fred Sandback and Richard Serra. It feels like a bit of a stretch, but some of the pieces (by Flavin, McCracken and Sandback) become all the more fascinating given so much room to breathe.