Archaeal Richmond Mine Acidophilic Nanoorganisms

This tomography paper about these tiny organisms is also available https://www.nature.com/articles/ismej200899.pdf

According to the paper these are 300-400nm across.

edit: If you assume a radius of 200nm and a carbon spacing of 3 angstroms, you can make a pure carbon perimeter that encloses it out of about 4200 carbon atoms.