Is Dawnguard going to be Teh Big Expansion (like Shivering Isles) or a rather small add-on (like Knights of the Nine) and an unknown Teh Big Expansion is still in the makings?

I think they said all the add-ons will be moderately big. Maybe(probably), not the scale of SI, but yeah.

It's going to be smaller than SI, that's what I believe that I heard. I also heard it would be bigger than the Point Lookout Expansion that Beth did for FO3 a bit ago. As I recall, that was maybe an 8-10 hours added if you wanted to do everything. It had a good amount of ground to cover, I might say it was about the size of one the smaller holds in Skyrim. Hoped this helped. Hope we more details today!

Size hasnt been said yet....

If their is no new Provence then I would say KotN, if we see a new land then SI. Least that is how I am going to judge it.