The Game Informer article says that he will be in the stage where you play Optimus (when the situation against Decepticons is looking dire, and you have to help buy time for the other Autobots). It also states that Metroplex will be around two to three times the size of Omega Supreme from WFC1.
So I don't know if we will only end up seeing part of him, maybe like his fingers flicking Decepticons into outer space.
I also wonder if this means he is bigger than Trypticon from WFC1 (since I am only playing Decepticon campaign from the first one, and not very far at all) ?