Jonah 3:3

‘So Jonah arose, and went to Nineveh, according to the word of YHWH. Now Nineveh was an exceedingly great city, of three days' journey.'

Jonah, now obedient to YHWH, arose and went to Nineveh in accordance with His word. If ‘three days journey is the correct translation we are then told of the size of Nineveh. It was a ‘three days journey', presumably in width. ‘Three days journey' is a set phrase that indicates a relatively short journey in contrast with a longer one of ‘seven days journey' (compare the usage in Genesis). It theoretically represented the distance the average person could travel in a relatively short period (‘three days' regularly means a short period). Some would take longer, others would do it in less. It is simply an approximate indicator of size. Taking into account the four sister-cities the description is quite reasonable, even if necessarily inexact.

It is, however possible that we should translate as ‘for a three day visit', with the indication being that visiting Nineveh could not be done in a day. It required following the accepted protocol. In Nehemiah 2:6 the word used here certainly means ‘visit' (the king was not interested in the literal length of his journey, but rather in the length of his visit).

‘An exceedingly great (or important) city.' or more strictly, ‘a city great/important to God'. In other words even God saw it as a large one, or even God saw it as important (possibly because it contained a large number of people - Jonah 4:11).