Size and Weight:
Paleontologists simultaneously know more, and less, about Iguanodon than they do about most dinosaurs--primarily because this classic ornithopod was one of the first dinosaurs ever to be discovered (in England in 1822, by Gideon Mantell) and only the second ever to receive a name, after Megalosaurus. (See 10 Facts About Iguanodon and a gallery of Iguanodon pictures.)
For example, one of the things we know is that there was definitely more than one species of Iguanodon--though the exact classification of this dinosaur is still a matter of some dispute. Fossils have been found as far afield as Asia, Europe and North America, but it's unclear exactly how many individual species ever existed, or how they were interrelated. This, in a nutshell, is why experts know less about Iguanodon than they should--because it was discovered so early, many unrelated dinosaurs were subsequently assigned to this genus, generating a huge amount of confusion that's still being sorted out today.
Classification difficulties aside, Iguanodon was a relatively gentle plant-eating dinosaur, despite its enormous size (twice as heavy as an elephant) and the scary-looking, foot-long spikes on the ends of its thumbs (which have provoked their own share of confusion, an early reconstruction placing the spike on this creature's snout). It's believed that these spikes may have been a means of defense against carnivores (perhaps including Utahraptor--see Utahraptor vs. Iguanodon - Who Wins? for an analysis of how this battle might have turned out), or they may simply have been handy adaptations for breaking open tough-shelled fruits.