According to the book of Jonah, "Nineveh was an exceeding great city of three days' journey." That obviously tells us that it was quite large, but in order to find out just how large we have to do a little digging. John Gill tells us that this distance most likely referred to its circumference. He says that Herodotus calculated a days' journey at about 19 miles, which would make the total to be 57 miles. And, if he was to travel through the city preaching repentance, his journey may not have been in a straight line. In any case, it would take Jonah three days to do what he was going to do while walking through Nineveh.
According to Wikipedia:
At this time, the total area of Nineveh comprised about 7 square kilometres (1,730 acres), and fifteen great gates penetrated its walls. An elaborate system of eighteen canals brought water from the hills to Nineveh, and several sections of a magnificently constructed aqueduct erected by Sennacherib were discovered at Jerwan, about 65 kilometres (40 mi) distant. The enclosed area had more than 100,000 inhabitants (maybe closer to 150,000), about twice as many as Babylon at the time, placing it among the largest settlements worldwide.