Here's one reason why we gave it a length of 40 feet, a height of 12 feet, and a weight of 12,000 pounds: hadrosaurs from Mexico tend to be bigger and stronger than those from US, China, Russia, Canada, and Mongolia. Not a bad theory, wouldn't you think?
I don’t know where the 12 meters estimates came from either. The article I read about its discovery just said that hadrosaurs could reach 12 meters (obviously referring to Edmontosaurus, the Asian Saurolophus and such), and that this taxon’s tail is 5 meters and that the body itself would be just slightly longer than that, so I assumed it was around 10.5 meters long? Thanks for finally providing some actual estimates.