By way of Wookiepedia,
According to the Star Wars: Incredible Cross-Sections fact book, the first Death Star in A New Hope was 160 kilometers in diameter. According to the Inside the Worlds of Star Wars Trilogy fact book and detailed scaling of the station in Return of the Jedi, the second Death Star was 900 kilometers in diameter. Some Expanded Universe sources state much smaller figures—120 kilometers for the first Death Star and 160 kilometers for the second—however, most of the evidence argues for the larger sizes.
-NJM