Mirabel airport [https://en.wikipedia.org/wiki/Montr%C3%A9al%E2%80%93Mirabel_...] probably was a contender. The passenger terminals were eventually torn down recently, but cargo flights continue to use it.

EDIT: "By surface area, it was the largest airport in the world that had ever been envisioned, with a planned area of 39,660 hectares (396.6 km2; 98,000 acres)"