These flightless birds are under strong selective pressure to be able to feed on high up conifer needles, and avoid predation from T-Rex sized predatory flightless birds. The forests the birds are evolving in are generally more open than those of our own Earth (due to being dominated by megafauna), but not as open as ones dominated by sauropods.
Importantly while these birds have many similar adaptions to sauropods, I've heard height is a much more significant limitation than total size, and that sauropods held their heads much lower most of the time than is often depicted. So with only tail feathers and not an actual tail it seems like these birds will be limited in how much their neck can extend horizontally compared to a sauropod.
Your answer should include some calculations and/or citations about the relevant physiological constraints on tail-less avian biped height. Particularly when it comes to getting enough oxygen to their proportionally tiny brains.
This is relevant as it seems like how high an animal can pump blood above its heart is actually fairly controversial: https://en.m.wikipedia.org/wiki/Sauropod_neck_posture