“Likely, when we close our eyes and think of a small space, it’s white walls, no lighting and bad storage,” said Nate Berkus. Mr. Berkus, the celebrity interior designer, was standing in a bright, cabinet-outfitted bathroom that he designed for Apartment Therapy’s “Small/Cool NYC,” a pop-up exhibition in Sunset Park, Brooklyn. The event, which is running on consecutive weekends this month and will conclude on Sunday, Oct. 29, is halfway between a showhouse and a showroom. More than a dozen 10-by-12-foot room schemes — imagined by creators as fresh as Maitri Mody, a fashion influencer who posts on Instagram under…