Using the footprints of parking lots and garages drawn into OpenStreetMap as a data source, the area of land in Chicago occupied by parking lots and garages is 247,539,968 square feet. (The data was exported using HOT Export Tool; you can replicate my export.)

That converts to:

  • 5682.712991749054 acres
  • 8.879274566670646 mi^2 (square miles)
  • 22.99721555608581 km^2 (square kilometers)
  • ≈ 0.26 × area of Manhattan (≈ 87 km^2 )