48 states of the USA (except Alaska and Louisiana) are subdivided into counties. All counties differ in number and size: Texas, for example, has 254 counties, while there are only 4 of them in Delaware. Anyway, the largest US county by area is located in the most populous state, California. San Bernardino County has an area of 20,105 square miles. Being the biggest county, it's also larger by area than some US states and even other sovereign countries. Moreover, San Bernardino County is bigger than New Jersey, Connecticut, Rhode Island and Delaware combined!

More Info: en.m.wikipedia.org