I was bored and thinking how many hitpoints various creatures would have, and was trying to think of the biggest ones out there. But ones that grow, not constructed ones.
The Dune worms are pretty big, but I know there are much bigger but I just can’t think of them.*
No comic books though, The stupid wang-contests to make something badder than the other guys, and you end up with Sentient universe beings and crap.
*the Star Trek Giant space Amoeba is just to stupid to think about
Stanislaw Lem’s Solaris features a planet that is a single gigantic life form. Probably something bigger out there, though. I’m not sure why you think “stupid want contests” would be unique to comic books, for this kind of thing.
In a semi-sequel to his novelization of the Star Trek: The Animated Series episode “The Eye of the Beholder,” Alan Dean Foster wrote about the jawanda, a gigantic but wispy space-tissue kind of critter which lived in interstellar space and fed off background radiation. It avoided gravity wells at all costs. The Enterprise captures one using several small, independently-powered moons supplied by the Lactrans, a very technologically-advanced race which collects alien lifeforms. The moons, properly aligned, form a gravity well from which the jawanda cannot escape (I know, I know). A preposterous but fun story.
In terms of mass, a jawanda is relatively small compared to Lem’s living planet, but in terms of wingspan, there’s no contest. 
“Clovie” from the Cloverfield movie was pretty large and nigh invulnerable. Does that count as Sci-Fi, though?
Similar to that, in Fred Hoyle’s The Black Cloud, the titular creature is an insterstellar cloud of sentient hydrogen, some 150 million kilometers in diameter.
The Star Wars Expanded Universe has the Yuuzhang Vong Worldships. Warhammer 40,000 has the Eldar Craftworlds, which are grown from wraithbone. They might not count as creatures, though, and I don’t think they’re as big as they sound.
The space slug from Empire was darn big. Jeff Russell’s starship dimension chart (click on “-2x”) shows that just the part we see coming out of the asteroid is bigger than a star destroyer. Wookiepedia says it’s likely to be 900 meters.
Huh. I never realized how big the ships were in Babylon 5. I’d have guessed that a Star Destroyer would be easily twice the size of the Agamemnon.
In the episode “Back to Reality” from Red Dwarf, Rimmer describes the Despair Squid as being the size of New Mexico. I suppose it’s just possible he was exaggerating a touch.
The “giant loogie” creature from Evolution was pretty damn big…
Hit -100x and check out the Super Star Destroyer.
Technically, I’m pretty sure it was a sentient ocean. I don’t think the solid matter of the planet and its core were part of the being.
That’s not the case with the Earth itself in Arthur Conan Doyle’s When the World Screamed, about the sentient Earth’s reaction to having a drill stuck deeoply into it. Possibly the stupidest Edward Challenger story.
Marvel comics had Ego, the Living Planet (originally in Thor, although he branched out). Just the opponent for the planet-eating Galactus.
A budong (from Farscape) is really big.
Gaea in John Varley’s Gaea Trilogy is a ring 1,300 kilometers across.
The Spheres in The Shattered Sphere are both organic and machine ( the aliens in the series, the Charonians, make no distinction between the two ), and are living Dyson Spheres.
The Anabis of The Voyage of the Space Beagle is a vaporous entity that fills it’s entire galaxy.
I’m not sure on their biggest size, but Farscape had the budong.
The Lexx was pretty big, but not the biggest. I think the Gigashadow on that show was bigger.
That chart that was linked to has the Vorlon Planet-Killer as being very big, much larger than the Lexx.
The avanc in China Mieville’s the scar was hefty - and brilliantly realised. Nothing planet sized, but a few square miles in size.
I think it was in one of the Flinx books by Alan Dean Foster. A creature called the Vom. Pretty damn big. Maybe someone else’s memory is better.
In Asimov’s later Foundation books, he put forth the idea that the entire Galaxy functioned as a single living organism, similar to the way Gaia functions as a living planet. If that counts, it’s going to be hard to beat (though there’s surely something out there with a living parallel universe).
And Discworld is fantasy, not science fiction, but Great A’Tuin the Star Turtle is unquestionably a single organism, and about the size of a planet.
Bloodhype. Big, but not on the scale talked about in this thread.