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.
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.
There are 2,569 square miles in Lincoln County, Nevada.
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
The smallest county in Texas is Rockwall County, which is about 147 square miles in size.
Jackson County in Florida covers approximately 917 square miles.
Onondaga County, NY is 806 square miles.
Marquette County, Michigan is 3,425 square miles.
Darlington County, SC is 567 square miles.
Hennepin County, Minnesota is 606 square miles.
Tillamook County, Oregon is 1,133 square miles.
Fulton County, GA's area is 534.75 square miles.
Baldwin County, AL's area is 2,027 square miles.