In the cover article of Mishpacha (May 25, 2011), More Than Math and Marathons: How Mathematician Dr. Leon Ehrenpreis Became a Follower of Rav Moshe Feinstein, Dr. Ehrenpreis’s wife is quoted as saying (page 52):
While these accounts differ somewhat from each other, they are in essence presenting two arguments: 1) Dr Ehrenpreis corrected the estimates of the traffic tally of Ocean Parkway, and this was a cause for Rav Moshe zt”l’s objection to a Brooklyn eruv. 2) Dr Ehrenpreis proved that Brooklyn was less than twelve mil by twelve mil in area, and therefore, Rav Moshe argued that the entire population of Brooklyn would be included in the tally (of the three million). This was grounds that, according to Rav Moshe, Brooklyn would be classified as a reshus harabbim.
Moreover, this argument is factually incorrect. Brooklyn is seventy-one square miles (without its inland water, which I think should also be included in the tally, and therefore would make it even larger). Twelve mil by twelve mil is sixty-four square miles (according to Rav Moshe’s shiur amah in regards to hilchos Shabbos). Consequentially, there is no doubt that Brooklyn is factually larger than twelve mil by twelve mil.