Okanogan County, Washington (data.census.gov) Okanogan County, Washington has 5,266.2 square miles of land area and is the 1st largest county in Washington by total area. Okanogan County, Washington is bordered by Skagit County, Washington, Chelan County, Washington, Lincoln County, Washington, Whatcom County, Washington, Douglas County, Washington, Ferry County, Washington, and Grant County, Washington.