It's a VR headset, angular resolution is fixed at 3800 px / 90 deg = 42px/deg. For a 27.5" virtual display half a meter away, let chord length = 700mm, radius ~ 600mm, distance to virtual display will be 488mm, and diagonal angle it occupies within view will be 71.4deg. with 42px/deg, resolution is ~3000px diagonal, or from (16x)^2+(9x)^2 = (3000)^2, x = 163.4, therefore resolution for that virtual monitor will be 2608x1470.

Other combinations of distances and virtual sizes can be calculated in the same manner.

How much of that “natural size” has been dictated by common monitor sizes, the limits of desk size/setup, and aesthetics?

My work setup as a 43” monitor set about 1 arm’s length away. I can just touch it when my fingertip if I stretch slightly. In a virtual space, I’d probably go slightly larger, maybe 45-48”, pushed a little further back.

I don't think I would want a monitor that big, since I think I would have trouble covering all of it without moving my head.

So then it's not "4k mirroring".