In studying any group of people in the past, it is natural to ask where they were active and how big their community was. For Rome's fourth-century Christians who are my subject, a good authority to answer both questions was and still is Richard Krautheimer. I remember him well, and two of his younger associates, Alfred Frazer and "Gus" Corbett. He and his team, over the course of forty years from the 1930s on, produced a set of five splendid volumes on the history and construction of the city's early basilicas. (1) I. PROPORTIONS