I love the first image.
As far as the Deacon goes well its not so much Xenomorph as its from Hybrid Human/Xeno Face Huger (Trillobite) so it is just as much if not more so Mammal.
So we wont see it grow as much in % as the Xeno, but if we look at Mammals well they can grow on average 4-5X larger when they are Adult so yes 15-20 foot for the Adult Deacon would seem about right.
Another thing is the Deacon is in no way as Alien or Scary as the Xeno, but as its part Mammal DNA from its early stages of gestation, we can look at how Cute a Puppy looks or a Lion and Tiger Cub they are not scary at all.... But look how larger and menacing a Adult Tiger/Lion and large Dog say Wolf/Rottweiler are compared.
Also as we are dealing with a Organism that is part Xeno, well the Xeno lifeform does go through some stages of change such as the Chest Buster does not have the appendages on its back that the Adult Xeno has and so its changes like this that could still occur to the Deacon when it is Adult.