basically skyrim is downloading the 3gb dlc I thought it would be smaller but i was kinda surprised by it and would like to cancel it and download later , I cant go off-line and start it because i wanna try some mods through the Workshop,any idea how to cancel the update?

Once you have started the download, whenever you open or hit play on Skyrim, you will get the download. Pausing it only allows you to access other games, not Skyrim.

See this

http://www.gamesas.com/topic/1343505-how-do-you-cancel-a-download-from-steam/