Biologists would say that there is proof that cephalopod-like Krakens, as opposed to Krakens with other body shapes, can achieve the dimensions and body masses of the largest known specimens of the giant squid, Architeuthis, and the colossal squid, Mesonychoteuthis hamiltoni, and there is no evidence about the probability that cephalopods might get any larger than the largest known specimens of those two species.
Most professional biologists scoff at claims that octopi or squid could get more than a little bit larger than the largest scientifically measured specimens. And there are claims that cephalopods get much larger than the largest scientifically measured specimens.
Here are links to two post discussing various non controversial, slightly controversial, and extremely controversial giant squid claims. Note that the second post is titled "Ludicrous Giant Squid Claims".
There is also the St. Augustine Monster, the remains of a large sea creature washed up at St. Augustine, Florida in 1896, which was probably a large lump of decaying whale flesh but possibly a giant octopus far larger than the largest known.
Then there is the story that a schooner was sunk by a giant squid in 1874.
Thus there is a big difference between how large it is proven that cephalopods can get and how large they might possibly be able to get according to various claims.
As for blue whales being the largest animals alive or possible, almost every single living blue whale was killed by whalers in the 20th century, and the largest ones would be likely to be noticed, measured and recorded. Thus the largest measured blue whales were probably the largest alive at that time.
But the sperm whales, Physeter macrocephalus, were not slaughtered so thoroughly in the 19th and 20th centuries and a much larger percentage of them were never killed or measured. Physeter macrocephalus is noted for the males being extremely larger than the females. And there are many reports of males much larger than the largest scientifically measured ones. If the largest such reports are correct, the largest sperm whales, possibly members of a different unnamed Physeter species, would be at least as long as the longest blue whales, and would be heavier per unit of length, thus being significantly more massive.
Furthermore a fossil prehistoric sea reptile discovered in 2018 might have been nearly as large as a blue whale.
And if the first specimen discovered happens to be nearly as large as a blue whale, and if the species had a wide range in size, it is possible that the largest were much larger than the largest blue whales.
In the late 20th century and early 21st century many fossils of sauropod dinosaurs that seem to be larger than the largest previously known ones have been discovered. Thus there is little agreement which species was the current record holder at at any one time. So the sizes and masses of land dwelling sauropods are inching toward the sizes and masses of sea dwelling blue whales.
And that is not counting the sizes and masses estimated from the now lost fossils of the two controversial but possibly largest sauropods ever found. The largest lengths estimated for them are longer than blue whales, and the heaviest estimated weights are heavier than blue whales.
Amphicoelias fragillimus or Maraapunisaurus fragillimus, and Bruhathkayosaurus Matleyi.
So it is possible, though unlikely, that there might have been land animals as large as or larger than blue whales, and because it is a lot easier for sea animals to grow large than for land animals to do so, it seems possible that there could have been sea animals much larger than blue whales.
I note that there were probably sauropod dinosaurs much longer than blue whales, even if less massive than them.
The lion's mane jellyfish Cyanea capillata is the largest known species of jellyfish.
The largest recorded specimen was measured by Alexander Agassiz off the coast of Massachusetts in 1865 and had a bell with a diameter of 7 feet (2.1 m) and tentacles around 112 feet (34 m) long,2 although it was incorrectly reported by the Guinness Book of World Records that the sighting occurred in 1870 and that the jellyfish measured 120 feet long.
Tentacles 112 feet long would be lightly longer than even the longest known blue whale, and if two tentacles were pointed straight in opposite directions that lion's mane jellyfish would have stretched for 224 feet from tip to tip.
The bootlace worm, Linneus longissimus is a very slender marine worm that sometimes gets very long.
In 1864 a specimen washed ashore in the aftermath of a severe storm by St Andrews, Scotland, which was more than 55 m (180 ft) long,6 longer than the longest known Lion's mane jellyfish, the animal which is often considered to be the longest in the world. However, records of extreme length should be taken with caution, because the bodies of nemerteans are flexible and can easily stretch to much more than their usual length.
I note that in this question:
Anatomically Correct Leviathan (Intelligent Design)13
The answer of IndigoFenix about a thermosynthetic organism indicates that extreme size would be very useful for such a creature's metabolism. So if someone wants to "get Kraken" and design a really giant kraken, one with one end embedded in the deep see bottom and the other end rising above the surface like an islands, a miles high thermosynthetic kraken seems like the way to go.