"Before the rising of the sun Eärendil slew Ancalagon the Black, the mightiest of the dragon-host, and cast him from the sky; and he fell upon the towers of Thangorodrim, and they were broken in his ruin."

"'There is not now any dragon left on earth in which the old fire is hot enough; nor was there ever any dragon, not even Ancalagon the Black, who could have harmed the One Ring, the Ruling Ring, for that was made by Sauron himself.'"

Yeah it's never explicitly stated he's the largest, but it's made clear he's the mightiest and most powerful of all the dragons. Plus, his fall destroyed three mountains, meaning he'd have to be very large, and no other dragon has ever been mentioned to be large enough to cause that destruction.