Ego, the Living Planet must be in the running.
The Star Trek novel The Final Nexus had that, although it wasn’t organic so probably doesn’t qualify. There’s other living universes in fiction ( I recall one story that postulated a whole ecology of them ); it’s the “organic” part that disqualifies them.
In the Sector General novels, there is a life form that is at _least _ continent (and possibly world) sized.
Shatner’s ego?
The Strata Beasts; there’s several of them. From Major Operation, where they perform life saving surgery on one with capital ship weapons. 
Doesn’t qualify as it’s non-fiction.
Mentioned by me in post #12
Okay, then: Kirk’s libido.
The alien discovered in Jack McDevitt’s Cauldron. IIRC, its eye was described as being as large as Earth.
Larry Niven’s Old Mind permeates the observable universe. I don’t know if it qualifies under “organic”; it’s components are described as resembling interstellar dust.
The interstellar drive described in Harry Harrison’s very tongue-in-cheek Bill, the Galactic Hero essentially works by “blowing you up” to many, many times your original size, until you take up a good portion of the universe, then shrinking you back down in the vicinity of your destination. Bill describes seeing the Earth as a tiny marble in front of him.
So anything using that drive is huge, much bigger than solar-system sized. Although presumably you’re in some other sort of “space” and not subject to our normal physical laws when you’re in it.
Maybe the creature in Futurama’s “The Beast with a Billion Backs”?
I would nominate the Sun-Eater from The Legion of Super-Heroes comic books.
The Spectre could get pretty big, too.
Darn, sorry. I didn’t skip your post, either, read it and somehow missed that.
Well, in the nuWho “Doctor Who” episode 42, there’s a living sun.
I think it’s strongly implied in If the Stars Are Gods, that some stars, even our own, are sentient.
And DC has Mogo, the living planet.
The first Sun Eater was even bigger than the more amorphous canonical version.
What about the thing at the end of Men in Black?
Nobody thought to include the gigantic cell from the TOS episode “The Immunity Syndrome”? A single cell, tens of thousands of kilometers long, on the point of dividing? How big would the entire organism be when fully grown?