Journey to the Centre of the Earth · Jules Verne, 1864
How big is the Lidenbrock Sea?
~520km
Verne never says. On the map it's about 520 km across under a 16 km high vault, and that is a guess.
From the bookThe map's guessReal world
When Hans builds the raft at Port Gräuben, nobody knows how wide the sea is. It has clouds, wind and tides under a stone sky, lit by a pale electric glow.
The crossing doesn't help pin it down: a storm drives the raft for days and, with the compass flipped, blows them back to the shore they started from. So the 520 km width and the vault height on the map are guesses that fit the story.
Set sail from Port Gräuben