The relationship between time and distance is presumed to be a system constant, which we named c.

So, a galaxy-sized lifeform would take a very long time to experience stuff. It takes a tiny but measurable amount of time to go from your brain choosing "Press button" to your muscles all that distance away firing to cause the button press, and then for the button press to have effect - at galaxy scale these periods would be much larger than all of human recorded history.

It'd have to be much more distributed in its ability to react, like octopuses arms being semi autonomous. They'll continue to pass objects towards the body even after being severed.

Or consider the humongous fungus: https://en.m.wikipedia.org/wiki/Armillaria_ostoyae

Sure, but it's not clear in this case whether say the human species should also count as a single "organism". We don't understand very much about the octopus, which is a healthy reminder of why I shouldn't even speculate about alien life which would almost unavoidably be much stranger than an octopus - but we feel comfortable asserting that the "semi autonomous" limbs of the octopus are not distinct in the way that say, my friends Chris and Caroline are distinct people. So if this galaxy sized organism consisted of smaller units with similarly distinct properties, I think we'd say that's not a galaxy sized organism that's a culture of individuals.

Good point. Of course this presumes that we understand the physics at that scale, and that there's nothing akin to a quantum tunneled nervous system, etc.

think it's relative upward and downward in scale. an entity at universe scale might be moving similar speed to us in our 3d space

No, I assure you that the constant is not concerned with scale, we're easily able to check that. A bigger device does not make this constant larger or smaller, you may be able to get more accurate results but that scale is unaltered.

Maybe as an eventually consistent life form using extremely slow message passing. Though gravity becomes a major factor that would limit the size unless it’s incredibly sparse.

One of my favorite episodes of Love, Death, & Robots is “Swarm”. Worth a watch.

Not so early in the universe age, but who knows what happens in 10^10^10^10 years. Also organisms consume energy, but mechanism of consumption of some ultra massive central quasars is beyond my imagination (I know Marvel has Hunger character but thats not the level of detail and logic I mean).

There is a great classic sci-fi book that explores this: https://www.amazon.com/Black-Cloud-Fred-Hoyle/dp/0140014667