In any case, you're referring to the size of the molecule, which isn't what determines whether a particle is a nanoparticle or not. A crystalline solid is a network of repeating unit cells, which in the case of S8 can be thought of as individual molecules. The particle size corresponds to the number of these repeating units that are connected before the particle is terminated. So even though the S8 subunit is very small, the particle can still be enormous (as far as particle sizes go, that is).