While it seems like one of the (possibly) reality-supporting towers would take the title of tallest Building on Tamriel (White-Gold, Crystal, Direnni), what about the largest building?
Of course, this isn't as easy a question as it seems. Buildings can be measured in different ways, such as by footprint, volume, floor space, etc. Let's stick to footprint (area on the ground) and volume (a multiple of length, width, and height in some manner). And lets stick to external geometries, as the insides of buildings don't usually match well with the outside in TES (or most other) games, and many buildings on the inside are best described as a series of tunnels which connect widely separated rooms. Good for adventuring, bad for many other purposes. Also note that I don't know if it is a good idea to count structures in Oblivion or other dimensions, as not only does space not necessarily work the same way there, none of them are actually on Tamriel.
Going by ESO, I would think that it boils down to three likely candidates:
1. White-Gold Tower (by volume)
2. Tribunal Temple (by floor area)
3. Palace of Vivec (by either one if not both)
I am sure there are ways to accurately measure the geometry using the models, but I unfortunately lack the time and means to do so. There's also the matter of in-game appearance vs. lore, as well as the fact that things can't always be kept consistent between games (different modelers, different engine, etc). Sometimes, this inconsistency is deliberate, like the difference between Mor Khazgur in Skyrim (a small camp made of wood), and Mor Khazgur in ESO (a gigantic stone arena many times larger than the camp.)
A few other candidates for the title, by one measurement or another:
- Scarp Keep (the Orc king's palace in Wrothgar)
- One of the monuments in Summerset (Royal Palace, Crystal Tower, Monastery of Divine Serenity, College of Sapiarchs)
- One of the castles in High Rock or Cyrodiil or possibly a xanmeer in Black Marsh
- A Dwemer structure, possibly Understone Keep in Markarth.