A couple of things:

#1. "seen from orbit" needs to be qualified: we must know the height of that orbit for the statement to be meaningful.

#2. The optical theory that is usually cited, and from which a calculation can give the minimum size of an object that can be resolved by the eye, refers to roughly circular objects. Because of the physival properties of light, long extended objects can be visible even if their width is apparently below the threshold for resolution. A common example of this (for astronomers) is the visibility of the Cassini division in Saturn's rings in a telescope whose aperture is theoretically unable to resolve its width.

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Stephen