Particle Physics is collectively an effort to study the exciting world of subatomic particles and the nature of their interaction. By subatomic we mean anything that happens within the atom or below and not above. The implications could cover as much above, as it would be entailed by the precincts of natural laws.
eg If a process corresponds to as big a size as is a micro-gram, its evidently not subatomic size in length dimension. The subatomic size by its definition of length scale would correspond to a femto-meters. But the given process is subatomic, while the result of having a size of micro-gram would not be.
Hence while the size of the subatomic entity can roughly be put by a femto meter, nonetheless a particle of the size of pico-meter might find relevance in the study of subatomic processes due to such eerie connections. For another matter a micro-gram is the unit of mass and not that of magnitude of distance.