why didn't they reveal this in the film, although awesome!

Remember, Indominus wasn't fully-grown in the film, she was mean't to be 50ft long if not in Height as well.

It says "''Over'' 40 feet" on the image. The official poster states she's 20 feet tall when fully grown.

Like Mjamnella says she would have been 20 feet tall and 50 feet long at full size. She was about 18 feet tall and 40 feet long in the movie. Rexy was 40 feet long and 16 feet tall according to the official website.. but the official poster puts Rexy at 16.8 feet tall and 44 feet long in the movie.

In the movie, I thought Henry Wu said she was going to be 50 feet tall when fully grown

There's still the debate on about whether or not that poster is actually canon

We need to figure out exactly WHO made the poster, and who made the book "Where Dinosaurs Come to Life" Because they both use the same material, with Rexy at 13.4m/44ft long and Indominus at 15.2m/50ft long. 

But are the opposable? I don't remember Iguanodon having such thumbs. The thumbs could be human. Brings back that crap from Sayle's script. Though, renaming the Dilophosaurus to "spitting frilled lizard" makes a little sense...