http://www.theouthousers.com/index.php/news/119342-skyrim-dragonborn-dlc-leak-part-deux.html
Start of the DLC:
Cultists of Miraak are searching for the false dragonborn (you), and when they find you they attempt to kill you. Assuming you kill them first, you find a note on them that tells you what they are doing and where they came from. You find their boat docked at Windhelm and proceed to Solstheim.
You don't have to complete the main quest to start the DLC, but the character of Miraak has different dialogue if you have already killed Alduin.
The size of Solstheim is about equal to the reach and has its own seperate map.
Player is about 30+ hours in and not finished yet, though said one could probably rush through the main quest in 10 hours. 