They said it would be Shivering Isles sized or bigger.
Erm, no they didn't. They said it would be bigger than Knights of the Nine, but smaller than Shivering Isles. They also said that it's worth 10-20 hours of gameplay, though they didn't specify if that includes both sides or just one, or if it includes additional sidequests or just the main faction lines. I don't have my HDD handy to check Oblivion's DLC, but I'd wager a guess that Dawnguard will be <1 GB in total file size. If I recall correctly, SI was only around 1.1 GB, and that was a completely new land mass with tons of dungeons and such. Seems like Dawnguard will have far fewer new locations, with the Soul Cairn being the only confirmed new land mass.