Judging from Google Earth (by copy-and-pasting Google Earth polygons into this website), the total built-up area of Nineveh in the late 8th century BC was just over 1800 acres-larger than that of than any other Near Eastern city at the time. George Rawlinson seemed to have been correct in his estimates of Nineveh’s size and population (at just under 100 persons per acre, roughly 175,000 persons; roughly a tenth of the population of modern Mosul). The population of the walled city of Nineveh in the 8th century BC was nearly 150% that of the entire Kingdom of Judah during its best days (the period 720-701 BC). The 120,000 mentioned in Jonah (100% of the population of the Kingdom of Judah during its best days) also seems a reasonable slight under-estimate for the population of Nineveh in the Late Iron Age. Even the great city of Kerkenes in Anatolia seems to have been only 613 acres in area. Jerusalem in the Late Iron Age was 162 acres. The city of Calah was 917 acres. Khorsabad was 760 acres. Babylon was 1030 acres.
The only pre-20th century cities of Mesopotamia I know of that exceeded the size of Nineveh were Samarra and Baghdad, the latter coming in at nearly 3000 acres inside its city walls.