Old lore was Marines were average 7ft unarmoured, around 8 with it then now primaris are noticeably bigger (Alpha Legion story makes that point, Custodes too) than before so probably 8ft unarmoured plus majority of the Imperium sees them as almost mythic at times which is where the trans human dread normal people experience is triggered by how fast they react or move for said size
Exactly. It's not just that they're tall. They're generally just larger than anything that looks human should be, much, much faster than anything that size has a right to be, and their proportions just feel off to unaugmented humans. Dan Abnett is actually pretty good at communicating how Space Marines look to normal people. He described one Ultramarine in
Know No Fear as being "tall as one big man on another big man's shoulders, broad as any three muscle-heavy athletes, his bulk augmented by the gleaming ceramite plate of Praetor-pattern armour."
Then there's this snippet from
Little Horus:
"The sight of an Adeptus Astartes was one thing: taller and broader than a man could ever be, armoured like a demigod. The singularity of purpose was self-evident. An Adeptus Astartes was designed to fight and kill anything that didn’t annihilate it first. If you saw an Adeptus Astartes, you knew you were in trouble. The appearance alone cowed you with fear.
But to see one move. Apparently that was the real thing. Nothing human-shaped should be so fast, so lithe, so powerful, especially not anything in excess of two metres tall and carrying more armour than four normal men could lift. The sight of an Adeptus Astartes was one thing, but the moving fact of one was quite another."
And this:
"Though there was no disguising his inhumanity, especially in this bared form. Apart from the sheer mass of him, there was the overgrown gigantism of the face, that particular characteristic of the Astartes, almost equine, plus the hard, taut shell of his rib-less torso, like stretched canvas."
