what's the biggest dungeon(S)?

In Daggerfall - Direnni tower.. (but any dungeon in DF is enourmous, you will get lost in dungeons on simple fetch quests)

Morrowind - The whole undercity of Mournhold is one huge dungeon encompassing sewers, a dark brotherhood base, several tombs and a daerdric shrine.

Oblivion - Ebrocca (SI) Cann (SI) and sundercliffe watch.

Probably something in daggerfall, everything there is huge!