At the present time quarks are considered "fundamental", ie. the smallest quarks (up and down) don't decay into anything else, although they do transform into each other.
Smaller than quarks are various leptons, such as electrons and neutrinoes.
In terms of spatial size, the quarks and leptons are all considered to be pointlike particles in the Standard Model, so one can't say that any of them is the "smallest."
In terms of mass, the neutrinos are the "smallest" of the leptons, followed in order by electrons, muons and taus.
I know there's something that is smallest because it would eventually be nothing if there wasnt a limit. Unless negative mass is what I think it is, then there would be no limit to how small something could get. Is negative mass what I am thinking it is? Please explain negative mass.
I thought negative mass actually meant like twice as small than positive mass, sorry yall. So neutrinoes, quarks, and the like are the smallest, thanks yall.
Preons are hypothetical particles which would be constituents of quarks and leptons. However there is no experimental evidence to support the theory. You can get a detailed history of the this theory from wikipedia.