i'm not quite sure what makes one particle smaller than another, but i'd think that the photon or the electron or (my favorite) neutrinos would win.
silverpig: neutrinos, as of 2000, fluctuate between something like tau, muon, and electron states. the easiest way to think of this (and i'm really pissed because i wrote a paper on it and now can't find the paper) is as such: you have three pendulums (pendula?) and they're all connected via rubber bands. You start the one a-swingin' and before long the second one is swinging and then the third one, and if you get it just right you can transfer all the energy from one to the other to the other so the first one is at nearly a dead stop. As was explained to me, that's how to think of the various 'types' of neutrinos.
interesting sidenote: the guy who taught me this is looking for supersymmetry. For every particle there's an antiparticle, and he thinks that for every force particle there's a gravity particle (i think; it was a while ago) and for every gravity particle a force particle. For instance, some day somebody's gonna find a photino.
I'll ask somebody who knows more than i about this, and try to reply with a real answer.