The island of France was about twenty miles long, five miles across the middle, sandy and crescent shaped. In fact it seemed to exist not so much as an island in its own right as simply a means of defining the sweep and curve of a huge bay. This impression was heightened by the fact that the inner coastline of the crescent consisted almost entirely of steep cliffs. From the top of the cliff the land sloped slowly down five miles to the opposite shore.
Five miles across the middle
Does this mean that the middle portion of the island is 5 miles long? Or 10 miles long that is 5 miles on either side of the middle point? Is "5 miles across the middle" the same as "the middle is 5 miles across"? Because when I say "my house is 5 miles across the road" it means it (house) is 5 miles after I've crossed the road.
In fact it seemed to exist not so much as an island in its own right as simply a means of defining the sweep and curve of a huge bay
I didn't get this at all. Isn't "in its own right" used for abilities?