Skyrim's game director and producer Todd Howard jokingly stated in an interview with the Norwegian magazine Gamer.no that
The game is actually too big.
Not that the map will be much bigger than Oblivion's. According to the interview the maps are about the same size. Howard said
If you put the maps on top of each other it is about as big as its predecessor. But Skyrim is different because of all the mountains that form our routes.
The terrain makes it smaller in practice, but you spend more time trying to move through it because they often can not be crossed. So it probably seems bigger.
He also said that there will be six or seven different types of environments, which should provide quite a bit of variation in the game world. The image to the right shows one of these environments. Howard stated
You have already seen the spruce forests and mountain-ranges. We also have grassy tundras, a volcanic tundra, a forest in the theme of autumn and a glacier.
Concerning dungeon, Howard stated that there will be around 120 dungeons in the game. He also said that there will be more diversity among the dungeons/caves in Skyrim than there was in Oblivion. Dungeons and caves in Skyrim will range from ice-coated to vegetation-covered and will all maintain some degree of uniqueness.