The total area for the county of San Diego in California is 4,526 square miles. Land makes up most of this area covering 4,207 square miles while water covers just 319 square miles of the county.
San Diego is 372.1 square miles in size according to Wikepedia.
324.3 square miles.
Jackson county is 100.6 square miles
There is no strict definition of "Southern California." But if you want to count LA County, San Diego County, Orange County, Riverside County, San Bernardino County, Ventura County, and Santa Barbara County; then Southern California is about 40,892 square miles. If you add in Kern County, San Luis Obispo County, and Imperial County...then Southern California is about 56,512 square miles.
El Cajon, CA, population 92,487, is located in California's San Diego county, about 12.6 miles from San Diego and 12.8 miles from Chula Vista. Reference: http://www.idcide.com/citydata/ca/el-cajon.htm
Marquette County, Michigan is 3,425 square miles.
Onondaga County, NY is 806 square miles.
Darlington County, SC is 567 square miles.
Tillamook County, Oregon is 1,133 square miles.
Lincoln County is 4 831 square miles.
Hennepin County, Minnesota is 606 square miles.
Rockwall County: 129 square miles.
Fulton County, GA's area is 534.75 square miles.
Baldwin County, AL's area is 2,027 square miles.