In the first teshuvah quantifying how many people would be required to live in this twelve mil by twelve mil area, Rav Moshe stated (ibid., 4:87) that since in the past eruvin had been erected in cities with populations exceeding shishim ribo, one could not classify a city as a reshus harabbim solely on the basis of the existence of a population of 600,000. He then added that although the actual number of inhabitants could possibly vary according to the city, in Brooklyn it would most likely require four to five times shishim ribo. In the final two teshuvos which followed regarding Brooklyn we see that Rav Moshe codified his chiddush that the requirement is, "just about 3,000,000 people," (ibid., 5:28:5) or, "at least five times shishim ribo," (ibid., 5:29) which could amount to even more than 3,000,000 people. Consequently, in the Chicago eruv pamphlet (West Rogers Park Eruv, 1993 p. 23) it is stated that Rav Dovid Feinstein shlita was in agreement that according to his father's shitah there must be a minimum of 3,000,000 people in order for the city to be defined as a reshus harabbim.
It’s important to note that Rav Moshe maintained the above only regarding shsihim ribo as it applied to a city. However, with regard to a sratya [intercity road] Rav Moshe stated (ibid., 1:139:5, 4:87, 5:28:16) that the shishim ribo would have to traverse the road itself every day.