As a person who lives and has grown up with the metric system, lemme tell you: 2.10 meters is not inhuman. You notice, sure, but it’s not outside the human realm of possibility.
However, a more important question than debating inches and feet:
You are a citizen of the Imperium. You are an Empire fearing, loyal inhabitant of a planet. You’ve never seen another world, you’ve certainly never seen an Astartes and you do what you’re told, which is keep your nose to the grindstone, your mind free of doubt and your tongue chanting in the Emperor’s most blessed name.
You see a giant of a man on the streets one day. He’s walking alongside a (person of clearly noble birth, a member of the Machine Cult or a member of the holy church) down the street. He’s tall, covered in slabs of inhumanly perfect muscle, yet dressed in simple, unassuming clothing (possibly armed).
Do you A) assume he’s some sort of vat-grown worker/bodyguard/off-worlder/mutant or any of the other countless things you’ve heard but probably never seen in your life, clearly sanctioned due to the person who’s presence he walks in, and go on with your life, perhaps telling your friends and family of the strange being you saw today?
or B) assume he’s a holy Angel of Death, a direct child of the most holy Emperor, somehow removed from his holy vestments of war and sacred bolter of righteous fury, one of the angels that can not fall, a living bulwark against the ruinous and totally not existing but very heretical forces of chaos? Walking the streets of your hive dressed as a normal person? With a (noble, tech-priest, priest)?
I’m gonna say option A seems more reasonable.
Space Marines are, in the eyes of the average (and even most not-so-average) citizens of the Imperium, nothing short of mythical, divine beings. They are loaded down with religious dogma and put on a pedestal. They are perfect, unfailing, unstoppable war machines of divine righteousness and religious fury. They are immortal, invincible and infallible. They do not fall. They do not falter. They do not fear.
The idea that your average pedestrian (or even your average Noble, Planetary governor or Tech-priest) would respond to the sight of one outside their iconic armor with unflinching terror, instant recognition and screams of “AHH, FALLEN SPACE MARINE! CALL THE INQUISITION!” is about as reasonable as to assume people in Uglyville would respond to a Calvin Klein model with screams of “HOLY ANGEL! PERFECT SERVANT OF OUR LORD AND SAVIOR JESUS CHRIST! CALL THE COPS!” even if said model really was an angel.
In my world, the only way you’re likely to solicit that response is if said marine is wearing a wreath of solid gold and followed around by a personal team of lightning specialist to provide a Halo light effect.
The rest of the time, yes, they will be noticed as the humongous slabs of muscle they are. But there are many, far more reasonable explanations for their inhuman bulk than “SPACE MARINE!” that people are likely to resort to first. Put them in the right company (your humans) or the right disguise (Servitor) and people will let it go. Yes, they’ll remember, yes they’ll gossip. But, unless someone specifically goes looking, that will be all it is. Rumor, hearsay and gossip. From, at most, a few thousand people. On a planet of billions.