There were even much larger pliosaurs. You surely all know the famous Liopleurodon from "Walking with Dinosaurs". They portrayed in the documentation a hypothetical specimen, the largest of its kind that ever lived, a leviathan of 25m. Sadly many many people confused things, and you can still read very often that this was no really uncommon length. But this 25m were only hypothetical and based on the assumption, that Liopleurodon could reach at least 18-20m. Later it turned out that this was not true, Liopleurodon grew only to about 11m or so, what is still the size of a very very large orca.
But several years ago the fossils of another pliosaur were discovered at Aramberri, Mexico. They belonged to a pliosaur which was 15-18m in length. It was most probably a different species than Liopleurodon, but has still no own name. It turned not only out that it had once a confrontation with a much bigger pliosaur which hurted it (those injuries were healed), but it was later even killed by another huge pliosaur whose teeth crushed its skull. This pliosaurs which attacked the "Monster of Aramberri" were about 1/3 bigger than itself, and the bones showed also, that the monster was still a subadult which had not reached its full size. So we had once pliosaurs which had really average lengths around or even over 20m, bigger than sperm whales.