There isn't really a good notion of the size of an elementary particle. What's really being looked for is particles with certain higher (mass) energies. Now, why do we say that higher energies allow us to probe smaller scales? Well, at small scales(/short times), quantum mechanics allows these higher energy particles to play a role, even if there's not enough energy to create an actual particle. The smaller the scale, the more do high energy particles come into the picture. Thus cranking up the energy allows us to see these particles at a larger (detectable) scale than they would usually occur.