Johnson County, Illinois, USA - 349 square miles.
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Harney County, OR is 10,226 sq miles (26,485 km²).
Idaho County is 8,502 sq miles (22,020 km²).
Tarrant County, TX is 897 sq miles (2,323 km²).
Yuma County, AZ is 5,519 sq miles (14,294 km²).
The largest County in Maine is Aroostook, measuring 6,829 square miles.