A Song of Ice and Fire
From there the skulls ranged upward in size to the three great monsters of song and story, the dragons that Aegon Targaryen and his sisters had unleashed on the Seven Kingdoms of old. The singers had given them the names of gods: Balerion, Meraxes, Vhaghar. Tyrion had stood between their gaping jaws, wordless and awed. You could have ridden a horse down Vhaghar's gullet, although you would not have ridden it out again. Meraxes was even bigger. And the greatest of them, Balerion, the Black Dread, could have swallowed an aurochs whole, or even one of the hairy mammoths said to roam the cold wastes beyond the Port of Ibben.
A Game of Thrones, Tyrion II
Although Drogon is said to be Balerion come again so given 200 years of life he could grow to be bigger than the previous dragon.
The Dothraki looked at her hatchlings uneasily. The largest of her three was shiny black, his scales slashed with streaks of vivid scarlet to match his wings and horns. "Khaleesi," Aggo murmured, "there sits Balerion, come again."
A Clash of Kings, Daenerys I
For why Drogon could still grow to be bigger than Balerion this quote gives enough details.
The squire Whitebeard, standing by the figurehead with one lean hand curled about his tall hardwood staff, turned toward them and said, "Balerion the Black Dread was two hundred years old when he died during the reign of Jaehaerys the Conciliator. He was so large he could swallow an aurochs whole. A dragon never stops growing, Your Grace, so long as he has food and freedom."
A Storm of Swords, Daenerys I
Game of Thrones
In the show it isn't exactly clear but from my looking at it Balerion's skull still seems to be larger than Drogon's.
Season 8 Episode 4, "The Last of the Starks", Dany is riding Drogon with Rhaegal in front, I tried to get as new an image as possible.
Season 7 Episode 2, "Stormborn", Cersei stands next to Balerion's skull.