I've seen some ArmA3 footage and weapon posturing seemed similar to how it is often done in games, where the weapon is held that high, that it obstructs the center horizontal line on the right side. No soldier does that in real life. Less of this, more of this. That means that no matter how large the sight is, the gun posturing should be adjusted to have the center horizontal line (height of the crosshair, spanning from the left to the right of the screen) always unobstructed. From a gamer point of view it's just not so good for awareness.
Generally I'd prefer to not even have the rifle pointed forward all the time, but held in low ready, where you have more unobstricted view and realistically need to bring up the sights to aim, or tap the fire key to quickly point the muzzle forward and shoot. Like this, or how this guy does it at 2:55. A crosshair wouldn't even be necessary, but I know that's just a dream and wont see the light of day in ArmA. Maybe a mod will finally make it.