Seems like the ultimate size would be limited by the reflectivity coefficient too. The speed of the photons makes for a whole lot of reflection events. The stats take over from there.
I found the idea of this experiment strangely compelling too.
This stuff is hard to begin to understand for most people I would imagine. That is generally true for me. For me, that all translates into an fun and interesting mode of conceptual thought akin to what happens with good sci-fi.
Mixing the concepts of gasses and light is novel to me as well, and I suppose that is what makes this one compelling.
I wonder whether this idea might be combined with how we have slowed the travel speed of light down many orders? Perhaps the materials forming the mirror box do that?
I am going to read this one again (during more normal waking concentration hours) and follow a few of the science rabbit holes the references will surely contain.