> Is there some minimal viable size for a 3d printer to be shrunk down to, aka, cellular level?

Great, not only chemicals, bacteria/virus, nuclear radiation, drones with guns can kill us all very efficiently, bot soon nanobots can devour us alive.

In principle you can pick-and-place individual atoms with an electron microscope. But if you think macro-scale 3D printing is slow, you'll be flabbergasted by how much slower this is.