Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Mĕgălŏpŏlis, is, f., or Mĕgălē pŏ-lis (acc. Mĕgălēn pŏlin), f., = Μεγαλόπολις or Μεγάλη πόλις, a city of Arcadia, the birthplace of Polybius.
Form Megalopolis, Liv. 36, 31, 6; 45, 28, 4.
Form Megale polis, Plin. 4, 6, 10, § 20; Liv. 32, 5, 5; 35, 36, 10.
Hence,