You would be wrong, as the blue whale is far larger than any known dinosaur and although a worm can be incredibly long, they are generally very thin. Certainly thinner than a blue whale.
Squrebear is correct. I think this chap ^^^ is the next biggest after the blue wale but I'm not sure, he just happens to be my fav dinosaur so I thought I'd shove a link up to him cause he's dead cool, (scientific term). There was a shark like creature actually who was massive too... I'll have a see if I can find him. But the Blue Wale is 100% biggest animal on the planet.