size chart of the Dragons of Middle-Earth - made by @contemporaryelfinchild.tumblr.com/

Ancalagon the Black

“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.”

Now in the Atlas of Middle Earth apparently there is a drawing by tolkien that would indicate that Thangorodrim was about 35,000 ft high. For comparison that is a good bit taller than Mt. Everest which is 29,029 feet. I cannot confirm this right now as my copy of the Atlas is on loan to a friend but that seems reasonable to me. 

Anyhow, that means that Ancalagon the Black must have been enormous to crush them under his bulk.

My guess at the size of some of the other Winged Dragons in the War of Wrath

“out of the pits of Angband there issued the winged dragons, that had not before been seen; and so sudden and ruinous was the onset of that dreadful fleet that the host of the Valar was driven back, for the coming of the dragons was with great thunder, and lightning, and a tempest of fire.”

While obviously none of these other dragons were as big as Ancalagon, I imagine that at least some of them were pretty huge by this description.

Glaurung
 “In the front of that fire came Glaurung the golden, father of dragons, in his full might; and in his train were Balrogs, and behind them came the black armies of the Orcs in multitudes such as the Noldor had never before seen or imagined. And they assaulted the fortresses of the Noldor, and broke the leaguer about Angband, and slew wherever they found them the Noldor and their allies.”

Now Glaurung is hard to place size-wise as Tolkien never really says but as Father of Dragons he must have been quite large and so Tolkien seems to describe him. He also looks quite large in Tolkien’s drawing but this is one that could definitely be argued. 

Drake of Gondolin

“but even as Tuor comes nigh driving the Orcs, one of those brazen snakes heaves against the western wall and a great mass of it shakes and falls, and behind comes a creature of fire and Balrogs upon it. Flames gust from the jaws of that worm and folk wither before it,”

So in Tolkien’s early writings on the Fall of Gondolin found in HoME there are a good number of dragons at the Fall of Gondolin and they knock down the walls and stuff. Again, pretty big.  

Scatha

Scatha the Worm was a mighty Long-worm of the Grey Mountains and one of the greatest Dragons to infest that range of the north. He was slain by Fram. Again, not much info so size reflects my best guess.

Smaug assessment of Smaug’s size was based largely off Tolkien’s drawings (picture below) 

So those are my sources, nice and simple. Again if you have anything more specific, send it my way.