The value of farmsand rural real estate for sale in Delaware's New Castle County recently on Land.com was $92 million. This represents almost 1,000 acres of land for sale in New Castle County. New Castle County ranks third in Delaware based on the total amount of acres of land for sale of all the counties in the Diamond State. Middletown featured the most land for sale. New Castle County is one of the ten largest counties in Delaware (494 square miles). It's located in the Northern Delaware region of the state.