He was big enough to level a city, given enough time. Like he did with Old Mournhold. And it took two of the Tribunal, at the apex of their power (given Dagoth Ur had not yet prevented them from renewing their connection to the heart), to defeat him. The guy actually smashed the roof off the Temple of the One, that's pretty impressive.
MD wasn't completely there. Sure, his avatar was mighty impressive, but the real MD is his realm. If you wanted to see the real MD in OB attack Nirn, we'd see a giant planet come and wreck everything.
I'd disagree with some of that, but that's another debate for another time in another thread.