San Bernardino County, California - 20,057 sq mi
*The smallest is Kalawao County, Hawaii - 12 sq mi
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San Bernardino County is the largest county in the contiguous USA at approx 22,000 square miles. It's bigger then some of the Northeastern states. There are some counties in Alaska that are huge, but Alaska is a country all to itself.
New Mexico: 121,589.48 square miles.
3,537,441 square miles.
The 50 states of the US measure a total of 3,794,066 square miles, or 9,826,630 km²
Alaska 663,267.26 square miles ... followed by Texas 268,580.82, California 163,695.57, Montana 147,042.40, then New Mexico 121,589.48.