What is the smallest elementary particle?
When it comes to elementary particles the concept of size is not defined.
Elementary particles such as photons, neutrinos, electrons and quarks all have known values for charge, spin, mass, energy and momentum. Size is however not defined.
Quantum mechanics describes elementary particles in terms of wave functions. They are often considered to be single point particles of zero size. If a particle has zero size and a mass then this implies infinite density. Mathematicians and physicists abhor infinities. Some people think that it is a meaningless question to ask what the size of an electron is.
Basically elementary particles are not solid objects in the physical sense. The only concepts of size we have are abstractions such as electromagnetic size.
One thing we do know is the size of a proton is