I am still unsatisfied with what I have read so far about the physical basis of metallic mirrors reflectivity. In particular, I am skeptical about the idea that individual electrons serve as mirrors by bouncing back photons, and I am tempted to think that there is somewhere an interface (e.g. air-metal), or at least something due to a collective action of electrons, producing a flat continuous plane of reflection to front waves (any photon-based explanation seems flawed to me). I am not merely asking about how many micrometers of metal are sufficient, but about atomic layout, down to the question of how conduction electrons might participate differently than other shells to reflection. Please do not answer if you cannot go down this level of detail, thank you in advance. Also, I am looking for plain English descriptions of physical processes, not math recipes (but feel free to combine). The point is to understand the minimum configuration of metal atoms (say silver, for example) that suffice to produce a mirror and why so.
EDIT: Using the popular definition of a mirror: a flat symmetric reflection of visible light, close to 100 percent. In other words, getting any light projected to the said mirror to reflect following usual optical rules of a bathroom mirror, although the question here is to know how small it can be.
EDIT2: PM 2Ring wrote a comment that is perfectly in line with what I am actually looking for: Yes, conduction electrons are the key, so I think you need enough atoms for there to be a "sea" of them, but I have no idea how small a metal crystal can be and still support a sea of conduction electrons that will give reflection that behaves as we observe with macroscopic mirrors.
So if someone could provide an answer to « how small a metal crystal can be and still support a sea of conduction electrons that will give reflection that behaves as we observe with macroscopic mirrors », that would answer my question.
EDIT3: unfortunately, the good stuff was moved from comments to chat. If someone wants to build on them and create a complete answer, be my guest. This question is about the particles level description of reflectivity. Unlike what KF Gauss says, it is not that I don't want an answer to depend on wavelength. It is rather that this is not sufficient to say the width of the mirror should depend on it without detailing the density and configuration of particles in between. An analogy, so to speak, is a Faraday cage. A Faraday cage is not a continuous sheet of conductor. So what about visible light, can reflection happen with a density of particles reduced to only cover the «corners» of a square sized to incident wavelength?