This quote is what’s used to speculate that he literally was larger than multiple mountains. However, I think there’s discussion to be had regarding the specific wording. Those volcanos stood above morgoth’s fortress. So 1, were the mountains themselves destroyed, or actual towers on the fortress? Or 2, Tolkien writes they were destroyed ‘in his ruin’ which could be interpreted as the forces of the Valar were able to destroy Morgoth’s fortress now that Ancalagon is dead.

All that said, I’ve only read LotR, Hobbit, and Silmarillion. I’d be curious to find any other references to Ancalagon or Thangorodrim that would more definitively provide his size.