Squid don't fossilize well (only the calcified "pen" and maybe the beak turn up), so we don't know as much as we'd like about their evolution, including just when they got to be really big. The modern giant squid can approach (maybe even, in long-tentacled examples, exceed) 18m. But was there a giant squid in the days of the dinosaurs? This paper says there was - and not only was it monstrous, it decorated its den with the skeletons of its victims. Not surprisingly, most paleontologists think this idea a bit preposterous. A piece of a squid's pen from 218 million years ago belonged, one paleontologist says, to a species at least as big as modern giants.
It's a very unlikely possibility, but it's fun to think about. Maybe the giant seagoing reptiles had equally giant rivals? We don't know when this "kraken" species, if it's being correctly identified at all, appeared, or how and when it went extinct. We just know there's a bit of a controversy here.