- Idaho County, Idaho has 8,477.5 square miles of land area and is the 1st largest county in Idaho by total area. Idaho County, Idaho is bordered by Missoula County, Montana, Adams County, Idaho, Wallowa County, Oregon, Lemhi County, Idaho, Ravalli County, Montana, Lewis County, Idaho, Clearwater County, Idaho, Nez Perce County, Idaho, and Valley County, Idaho.