There is usually at least a small "Day One Patch" for every game and sometimes there are larger patches at this time.
There is not any way to know the precise size of the day one patch until Skyrim's release date. However, once you have activated and installed the game, it is possible to change your settings so that your game is not updated automatically with subsequent patches.
Feel free to continue discussion of Steam, Steamworks etc., here: http://www.gamesas.com/index.php?/topic/1243801-unofficial-steamdrm-discussion/