What is the biggest dinosaur on Earth?

There are a couple problems with your question seeing as swimming dinosaurs didn't exist, and I'm not sure what you mean by gigantic world, but I will do my best to answer it. The largest aquatic reptile from the time of the dinosaurs was a carnivirous pliOSaur 15m long. It is unamed. The largest ocean going creature at the time was Leedsichthys, a massive fish 27m long.