Being a huge MW fan, I am pumped for this expansion! I can't wait to revisit some of the locations from TESIII, and I am hoping that the landmass will be quite extensive. Aside from stating that Dragonborn will be set in Solstheim, do we have any information about the landmass?

Well, Skyrim's scale is smaller than Morrowind's, so if bethesda wants to keep Solstheim's size consistent, it would be smaller now than it was in Morrowind. But Bethesda cold either disregard this and make it as big or bigger than the Morrowind Solstheim, or they could make it bigger using the excuse that the volcanic activity related to Red Year made the island grow.

I am hoping that we might even see some of the islands near Dagon Fel. I would love to see some of the daedric ruins from MW.

The southern part of Solstheim is now "ash wastes" and home to Ash spawns and netch/scribs. The Dunmer also reside in this area as Raven Rock is located there as well as Telvanni Towers. The place may seem smaller because of scaling or will feel larger due to slower running speed and grittier terrain w/ larger mountains. It took me 2 and a half minutes to run across Solstheim in Morrowind with a 300 speed stat. That probably puts Skyrim's Solstheim at around 5 minutes.

They will HAVE to make it larger.

The whole scale of the Elder Scrolls games has gotten completely ****ed.

Think about it; there also has to be a huge distance between Solstheim and Skyrim/Morrowind (an OCEAN, in fact)
Yes, you could swim there in Morrowind, but it took AGES. Either way, it felt really far away, and I hope they can bring that back for this.

Hopefully it is big enough that, if I want, I can stay there and not feel like I'm sitting in sandbox (not in the 'open-world' context, more in the sense of a Child's sandbox)