A combined around 1,000 acres were listed for sale in New Castle County recently; the combined value of all New Castle County farmsand rural real estate for sale was $105 million. New Castle County ranks third in the First State based on the total acres of land for sale among the three counties in the "Small Wonder. 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 Delaware.