Disclaimer: I have not programmed a complete game engine or worked extensively with levels inside existing game engins, but I do have experience making a terrain engine with XNA.
As far as the Oblivion vs Crysis map size goes, I'm going to take a guess and say that, per pixel in the height map, they are either using a different unit of measure, or a different size. A pixel on that map can be 10 meters x 10 meters OR it can could be 100 meters x 100 meters. It really depends on much you scale the height map.
Just by looking at the oblivion map inside the cryengine, it look's like you're several times bigger than you should be. I bet if you shrunk your character down, it would look a lot more normal.
Yes, you can make an open world in the CryEngine (See Crysis). You can make the maps any size you want. The great thing about using an existing engine that can handle those open worlds is that it's taking care of the scene for you. When I programmed my own terrain program, I had to create Octree's so that only the parts of the scene that were in front of my character were visible. It was a lot of work.
Hopefully that helps?
but crysis island is not explorable in all of the size
Eh? Wait what? I remember being able to go anywhere on the island. Perhaps my memory deceives me?