I've read some fan fiction dealing with the inheritance cycle, and some of the writers make Shruikan's size absolutely huge (far bigger than he appears in the 2006 film). We know that Dragons never stop growing, and that the largest dragon was Raugmar the Black. But we don't know exactly how big he was.
Is there any indication in any of the bigs how big Shruikan is, beyond "huge" or "very large"? I don't remember this being stated in the books.
This scale diagram has been officially sanctioned by Christopher Paolini:
It shows (from left to right) Firnen, Saphira, Eragon, Thorn, and Glaedr, with Shruikan dominating all of them. It's fan art, made by one EloiseS16, but Christopher Paolini has confirmed on Twitter that this drawing is accurate:
The dragons of the series to scale. Might make Fírnen a little smaller, but otherwise dead on:
Since the picture includes a human being (yes, that little smudge between Saphira and Thorn), you can judge for yourself just how big Shruikan is.
Never. Count. That. Film.
The 2006 film was not canon. The 2006 film did not follow continuity. The 2006 film, for all intents and purposes was a filthy money-grab attempt and should never be spoken of again.
ANYWAY, the books give a much better estimation of the fell dragon's size. If you've read the books, you'd know that the party eventually encounters Shruikan, and at first mistake his side for a strangely patterned wall in a massive, massive audience chamber.
I hesitate to say more, as it seems like you haven't read the novels, and I wish not to spoil anything. But read the books, they are wonderful and will answer this question eventually.
Edit: screw it, spoil'ahs ahead!
At the end of the final book, Eragon and co. make their way to Galbatorix's throne room. This room is friggin massive. One entire wall is covered in a decorative curtain. During the confrontation w/ Galby, that curtain...moves. It wasn't a curtain, but rather the wing of a massive dragon (Shruikan). Shruikan is freaking massive. He takes up a huge secondary-chamber to the massive throne-room.
Christopher Paolini answered this question on his Reddit AMA.
How big was Shruikan?
Biiiig. Big enough you could swim through the chambers of his heart.
(Source)
In the first chapter of Murtagh an anecdote is shared about the size of Shruikan.
Once a dragon was large enough, it became impossible to sit on like a horse. Shruikan—Galbatorix's mountainous black dragon—had been like that. Instead of a saddle, the king had installed a small pavilion on the hump of Shruikan's enormous shoulders.
To give proportions to Shruikan, it is observed that he is larger than Glaedr but not quite as large as Belgabad.
What struck him most was their size. A few of the dragons had been smaller than Saphira, but most had been far larger. The biggest he saw was a skeleton with ribs that he guessed were at least eighty feet long and perhaps fifteen wide at their thickest. The skull alone - a huge, fierce thing covered with blotches of lichen, like a rough crag of stone - was longer and taller than the main part of Saphira's body. Even Glaedr, when he was still clothed in flesh, would have appeared diminutive next to the slain dragon.
Inheritance (p.499)
So, indeed, Belgabad's ribs were 80 feet long and 15 feet wide. Also, just his head was about as large as Saphira.
As for Shruiken's size by comparison:
The sound was unmistakably that of a dragon walking on stone. But what a dragon, to hear its steps from over a mile away! ... How big is he? Eragon wondered, dismayed. Bigger than Glaedr, that was certain. As big as Belgabad?
Inheritance (p.608)
He was said to be about the size of belgabad, who had ribs which were 80ft long and 15ft wide. so overall he was pretty big
