Depends on the particle! Electrons are considered to be point particles with an infinitesimal radius. A proton, however, is made of three bound quarks, which al

Depends on the particle! Electrons are considered to be point particles with an infinitesimal radius. A proton, however, is made of three bound quarks, which all whiz around each other within a small but finite region. By bouncing things off the proton, as is done in scattering experiments, a notion of how large that region occupied by the quarks is can be determined.