Excluding Blackreach which would be the obvious answer... what is the second biggest dungeon?

Blackreach is its own area, if taking that into account, then the Forgotten Vale is a good choice. So is the Soul Cairn

Forelhost is a good candidate.

I would say Blackreach qualifies as a dungeon.

After that, take your pick of a few of the Dwemer ruins. Mizinchaleft comes to mind.

Forelhost would be my bet too. Though you can skip a lot of it if you pick a master level lock. Blackreach and Soul Cairn too.

Blackreach, Soul Cairn, Apocrypha, Deathlands, Shivering Isles. 5 reasons why i keep buying TES games

Bthardamz or Nchuand-Zel

I've heard of only Black Reach out of those. I thought I had discovered most of them in Skyrim. Clearly I have a few more discoveries to make!

Deathlands and Shvering Isles are in Oblivion, the previous game. Soul Cairn is in Dawnguard DLC, and Apocrypha in Dragonborn DLC.

These, but Alftand is pretty big, too.

Ahh That explains it! Thanks. I have only dipped my toes in Oblivion so far. Have yet to get into it (hate using keyboard, prefer controller).

The Cellar.

... below Anise's Cabin.

Where are the Deathlands in Cyrodil?

Yeh, I think Aftland is bigger. Mizinchaleft is the only one I typically can remember and spell correctly, so I listed it.

I think he means the plains of Oblivion during the MQ.

I've come across so many magnificent dungeons. I meant to record them but was always too lazy to get up and grab a pen and paper. Doh. I agree with the ones listed thus far.

Yeah, that's the common name of the plane the Oblivion Gates lead to. The different areas have their own names too.

Oh, my bad, it's Deadlands, not Deathlands http://www.uesp.net/wiki/Lore:Deadlands

Bthardumz was a long, hard fought thing. But the biggest... maybe Arkngthamz.

Ansilvund in the Vanilla game is a contender.

Forelhost would be my best bet.

Of the Dwemer ruins, I suspect Bthardamz is the biggest, but Irkngthand wouldn't be far behind.

Mzulft, Tolvald's Cave, Forelhost and Irkngtand comes to mind. Alftand is long too, but I've been through it so many times that it doesn't FEEL that long anymore.

To me, Blackreach consists of several dungeons (Reeking Tower, Silent City and so on).