There's a lot of fun (as well as science) involved in wondering just how big various types of marine life can grow.
Take sharks, for example - specifically, great whites. The maximum size of the great white is a recurring theme on every marine life website (and always will be: people sometimes seem ready to punch each other over a difference of centimeters), but the "McCosker-Ellis Limit" (there, I just named it) of approximately 7m/22 feet still stands for physically measured sharks. (Ellis was the first author on the book just cited: somehow, putting it the other way sounds better when I say it. No offense meant, Richard.)
However, I finally took the time to dig up a reference I'd used in my 1995 book Rumors of Existence, an article from Science where Dr. John Randall (who gets credit for being conservative based on his cutting in half the estimates for the extinct giant C. megalodon) reported that bites on a whale carcass indicated a shark of about 7.5m.
This terrific image, this Nat Geo article, and the paper they were based on looked at the biggest creatures of various types (it didn't look at all the orders or families: no one cares what the biggest cycliophoran is, no matter how important is it to science.) (OK, the answer is about 350 millionths of a meter.)
Everyone will get a bity of an eye-opener: I didn't realize (or had forgotten) that southern elephant seals could weigh a mind-boggling 5mt, or that a whale shark had been measured at 18.8m. On the other hand, the authors shrunk the giant squid, widely reported at 17-18m, to 12m: the longer figures, they believe, were based on bad measurements and the tendency of observers to stretch the tentacles on stranded specimens out far too much. I am still inquiring about the nearly 24m sperm whale: other sources put the record at 20.7m, ascribed to a bull killed in 1950. Ellis' book The Great Sperm Whale mentions that there are two teeth 28cm/11in long in a museum, where the normal length is 20cm/8 inches: perhaps they were extrapolating.
We think of invertebrates (besides the octopus and squid) as small, but there are jellyfish 2m across, barrel sponges 2.5m wide, and isopods (think of the biggest roach you ever saw, give it gills, and blow it up in size) 50cm long. Someone in the comments on the Nat Geo site noted the absence of siphonophores, which are longer than blue whales, but I suppose colonial creatures don't count as one unit.
As an extra note on jellyfish, cryptozoological literature often contains a description of a monster jellyfish weighing at least a metric ton washed onto the bow of the steamer Kuranda in the South Pacific in 1973: the steamer could not get free of it and had to be rescued by a seagoing tug, the Hercules, which washed the mess off with high-pressure hose. Despite claims a sample was scientifically verified to be a jellyfish, I've given up on this report: it all traces back to a newspaper clipping and nothing else, and all I can document from Web searches is that the Hercules, at least, was a real ship, and that doesn't get us very far. (Author Richard Weiner claims to have seen a 15-m jellyfish while scuba diving on the other side of the world, but that seems to be the only such report, and I set that one, too, aside.) I'm ready to accept around 2m as the max for a jellyfish.
The authors note these are maximum sizes, and the average is usually much smaller. But everyone, including me, is fascinated by the upper end. I went to the Whales: Giants of the Deep museum exhibit and watched children crawl through a blue whale's heart. For me, the amazement never ends