How big was Ancalagon?

[Aiwendil]

Actually, it is reputedly said in the History of Middle-Earth pt. IV, i.e. the Shaping of Middle-Earth (page 110, my Atlas of Middle-Earth says) that Thangorodrim (if they are indeed what is meant by the "towers", but I think they are, since they are referred to like that several times, I believe) were the highest peaks in Middle-Earth

The quote is from the 'Quenta Noldorinwa':

Feanor died in the hour of victory, looking upon the gigantic peaks of Thangorodrim, the greatest of hills of the world

"The world" was subsequently emended to "the hither world", presumably to allow for Taniquetil to be the highest peak in Arda.