The largest by far is Alaska with 663,267.26 square miles. The next largest is Texas with 268,580.82 square miles.
By population it is Los Angeles County, California (10 million +) and by area it is San Bernardino County, California (21,000 square miles). The largest "county equivalent" by area is Yukon-Koyukuk Census Area in Alaska (147,805 square miles). It is part of Alaska's Unorganized Borough and has no county seat. Its population is less than 6000.
The largest county in square miles in Kansas is Sedgwick County, with an area of approximately 998 square miles.
The largest is Berkeley County with 1,228 square miles.
Shelby County is the largest county in Tennessee by square miles, covering an area of approximately 784 square miles.
The largest is Nye County with 18,147 square miles.
Osage County is the largest county in Oklahoma, measuring approximately 2,304 square miles.
According to the 2010 U.S. Census, Los Angeles County.
Texas Counties and County Trivia: Texas has some large counties like its largest Brewster County at 6,193 square miles. Texas has no county that fits into the 25 largest counties (or equivalent units) in the US, with seven Alaska counties at 12,750 square miles each or larger.Pecos County at 4,764 square miles.Hudspeth County at 4,571 square miles.Presido County at 3,856 square miles.Webb County at 3,357 square miles.Val Verde County at 3,171 square miles.
It is Ware County with 903 square miles.
Keweenaw County - 5,966 square miles.
Worcester County with 1,513 square miles.