You've totally misinterpretated, it already is nine feet tall in the film. It's just instead of using the words "Covenant Alien" or the more scientific term "Praetomorph" the term "Stalker" is another name for easy reference. And I think the film is by far the best one to come out of the franchise in the past 28 years, concerning the Alien itself, I much preferred it in every way over any other version of the creature since the first three Alien films, apart from perhaps Isolation.