It depends on whether you measure by land area or by population. By area, Pike County in far eastern Kentucky is the largest, at about 788 square miles, followed by Christian County (about 721 square miles), Pulaski County (662), Hardin County (628) and Ohio County (594).
By population, Jefferson County, home to Louisville, is by far the largest, with an estimated 795,222 residents in 2025. It is followed by Fayette County (Lexington, about 329,751), Kenton County (about 175,779), Warren County (Bowling Green, about 149,375) and Boone County (about 145,316). Kentucky has 120 counties in all, and the smallest by both measures is Robertson County, at about 100 square miles and 2,325 residents.
Source: List of counties in Kentucky, Wikipedia (U.S. Census Bureau data)
Image: Kentucky counties map, public domain, via Wikimedia Commons.