I would imagine Narnia corresponds roughly to the eastern coast of Britain, if Lewis was anything at all like his peers, who tended to apply real-world geography to their fantasy worlds. Overall I'd say it can be imagined as a large island.

Can't be, Berserker. Well, unless you say that a continent is a large island. But you could say an ocean is a ginormous lake. Anyway, Telmar is to the West of Narnia.
Narnia's Northern Border: In SC, it seems like it is a river.
Narnia's Southern Border: We know that the Winding Arrow is Archenland's border with Calormen from HHB. Is the border the mountain ranges and Mt. Pike?
Narnia's Eastern Border: The easiest one- the sea!
Narnia's Western Border: Somewhere in the Western Wood, and on the other side is Telmar.
So, it's bordered to the Northern and Southern Marches by rivers, East the sea, and west a forest. I'd say it's a rough rectangle, with the Wild Lands to the west. And Calormen, I think, extends farther west than Narnia. But the whole thing is like Pangaea.
And, of course, it's flat!

The books explain that the world of Narnia is flat.
It's kind of like the argument with Earth back in the day before satalites. People thought once that the earth was flat and that we would go over the edge if we sailed too far.
With Narnia going east you'll head into Aslan's country. If you sail north or south then yeah you'll go off an edge somewhere. I assume.