Spock informs Kirk that the ship has entered the latest solar system in their search pattern, and we learn that it, like the last three, seems to have been totally reduced to rubble and dust. [...] Spock corrects himself, tells Kirk that this solar system has two intact planets.
As to the "Eater" itself...
[...]a kind of cylindrical "living atomic rocket" at least ten times the size of the Constellation, apparently from beyond the Galaxy, with a big posterior rocket and a great anterior funnel-mouth big enough to swallow a ship with a cluster of atomic blaster beams and tractor beams around the funnel, not a machine, but a living organism with a nuclear metabolism.
Then from the script received by the Star Trek offices April 5. 1967:
On screen, looming ever-larger as it closes with the Enterprise, we see the Eater: a great funnel of a mouth extended before its huge, almost endless metallic, undulating body, as if to devour the Enterprise, and a trail of what might be rocket exhaust dribbling off behind it.
Now we get an idea of its size... eight miles, or about
44 starships long; much bigger than portrayed in the episode...
Approximately eight miles in length, metallic sheen, and yet I sense that it's a living organism, not a machine....
[...]
[...]the only way to destroy it is to get inside its guts...right down that monstrous mouth at full emergency speed...smash its internal organs with the momentum of the shuttlecraft....
[...]
[...]Neutronium is a nuclear dampener, isn't it? That means the thing's guts must be made of something else, something more vulnerable.[...]
In the May 8, 1967 "final" draft...
[...]a thing ten times the size of a starship and far more powerful[...]
If he means length, Decker's description of the thing slightly would be maybe 10,000 feet long, or a bit shy of two miles. What we see in the finished episode—if scaled in profile views with the
Enterprise—measures to about 14 starships long or ~13,330 feet, so Decker's description would be about 20% smaller than portrayed, and less than a quarter the 8 mile length Spock gave in the previous draft.
But
then we then get Sulu's and Spock's estimates of its length, are even smaller than what Decker described. Here's the description of what it looks like and its size:
On screen, looming ever-larger as it closes with the
Enterprise, we see the Eater: a great funnel of a mouth extended before its huge, almost endless metallic, undulating body, as if to devour the
Enterprise.
Gigantic...thousands of meters long....
Metallic body...a large funnel-mouth...at least a mile long.
[...]
A most fascinating device...it's solid neutronium, and by isotope dating, it's at least three billion years old!