The Dark Lord Morgoth bred Ancalagon, called the Black, during the First Age, to be the greatest and mightiest of all dragons, and the first of the winged ‘fire-drakes’. Near the end of the long War of Wrath that pitted Morgoth’s hosts against the Host of the Valar, Morgoth sent Ancalagon, leading a fleet of winged dragons, from the fortress of Angband to destroy the Dark Lord’s enemies. So powerful was the assault of the dragon fleet that the host of the Valar was driven back from the gates of Angband onto the ashy plain of Anfauglith.
But Elendil ‘The Blessed’ in his powerfully hallowed elven airborne ship Vingilot duelled with Ancalagon for an entire day, until Elendil at length prevailed, pitching Ancalagon onto the triple-peaked towers of Thangorodrim, destroying both dragon and towers. With his last and mightiest defender slain, Melkor was soon utterly defeated and made captive.
Ancalagon was said to have been so large that he blotted the Sun out, even from afar. He was the largest of any dragon to appear in Middle-earth, even larger than Smaug. His length was unknown, though longer than Glaurung or any other ground dwelling dragons. Despite his legendary fire-breath, Ancalagon was reckoned to be insufficiently hot enough to melt the One Ring, and none of the lesser dragons could do so.
Ancalagon was possibly long-lived like other dragons Melkor bred. So large a dragon would have taken centuries to grow to his full size. The method of how exactly Elendil managed to battle and slay so titanic a creature is not explained.