What is the smallest particle?

Answer 1

This particle is the electron.

Answer 2

This is a difficult question because size doesn't really apply to particle physics, even less when one considers particle-wave dualities. Of all the particles that can be considered as "matter", muon neutrinos (νμ) are currently understood as the lightest, with a mass of about 170 keV, or around 3.311 x 10-31 kilograms.

On the other hand, forces are also carried by (non-matter) particles, photons (mediate the electromagnetic force) and gluons (mediate the strong nuclear force) are both known to have zero mass.