This list is a bit old but interesting:  Catholic Dioceses in the USA listed by size

Rockville Centre is # 8 and our neighbor and 'parent' Diocese of Brooklyn is #5.  We used to be part of the Brooklyn Diocese so if RVC were not created, the combined Diocese would be #2.

Here are the top twenty:

#	Catholics	Population	Percent Catholic	Diocese	Year	Source
1	4,174,304	11,096,200	37.62%	Los Angeles (Archdiocese)	2004	ap2005
2	2,521,087	5,602,418	45.00%	New York (Archdiocese)	2004	ap2005
3	2,442,000	6,104,000	40.01%	Chicago (Archdiocese)	2004	ap2005
4	2,077,487	3,970,026	52.33%	Boston (Archdiocese)	2004	ap2005
5	1,826,592	4,694,705	38.91%	Brooklyn	2004	ap2005
6	1,486,058	3,872,783	38.37%	Philadelphia (Archdiocese)	2004	ap2005
7	1,481,866	4,441,551	33.36%	Detroit (Archdiocese)	2004	ap2005
8	1,428,563	2,753,913	51.87%	Rockville Centre	2004	ap2005
9	1,319,558	2,835,594	46.54%	Newark (Archdiocese)	2004	ap2005
10	1,170,480	3,000,000	39.02%	Orange in California	2004	ap2005
11	1,041,123	4,972,544	20.94%	Galveston-Houston (Archdiocese)	2004	ap2005
12	1,017,196	3,556,631	28.60%	San Bernardino	2004	ap2005
13	930,379	3,041,195	30.59%	San Diego	2004	ap2005
14	930,352	3,371,300	27.60%	Dallas	2004	ap2005
15	904,000	4,755,500	19.01%	Seattle (Archdiocese)	2004	ap2005
16	888,004	1,044,711	85.00%	Brownsville	2004	ap2005
17	856,783	4,036,799	21.22%	Miami (Archdiocese)	2004	ap2005
18	815,719	1,966,067	41.49%	Pittsburgh	2004	ap2005
19	812,675	2,852,331	28.49%	Cleveland	2004	ap2005
20	780,925	2,000,235	39.04%	Trenton	2004	ap2005