What would you say if I said “The Israelites could not have crossed the Sea of Reeds (often translated Red Sea) because there was no wood in the wilderness from which they could have built rafts”. Furthermore, they couldn’t have lived in the wilderness for 40 years as there would be no water for them to drink.
But here, the bible is very specific. They didn’t cross in rafts; The sea was split for them. They drank from the well of Miriam and they ate mon (mannah).
So – If you think there is no g-d, or there is but he didn’t write what we call the bible, then it really doesn’t mater whether Jonah was swallowed by a great fish.
If you think there is a g-d, and he wrote the bible, then – well – that still doesn’t answer the question of whether Jonah is literal, but you certainly can’t answer it by talking about the physiology of fish.
Look at Bilam’s donkey. I’ve never heard an interpretation that says it didn’t actualy speak to Bilam. I’m sure if we ask a few verteranarians, however, you’ll find that most donkeys are incapable of intelligent conversation.
Now – with the size of Nineveh: The size of the city does not necessarily indicate how many people are in it. You’d be suprised that the largest city in the US isn’t LA. I remember it being some low-population city in Florida, I think (can someone help here).
There are a lot of very interesting interpretations of Jonah. I’ve heard it as an allegory of the journey of the soul. I’ve heard it as a battle between g-d’s attirubtes of Justice (truth) and Mercy. Jonah is called Yonah ben Amitai which means Jonah son of Justice. He is upset that Nineveh will receive mercy even though their repentance is temporary, and embarassed that the Jews will not repent and Israel will eventually be destroyed by the Asyrians.
Anyway – My point is that you’re reading parts of the bible as if there’s no g-d.