How large was Nineveh, according to Jonah 3?

Answer: A three-day journey

Nineveh was an exceedingly great city, requiring a three-day journey. Jonah began by going a day's journey into it.

“This time Jonah got up and went to Nineveh, in accordance with the word of the LORD. Now Nineveh was an exceedingly great city, requiring a three-day journey.”— Jonah 3:3 (BSB)