According to my research, quark is the smallest particle. So far there's nothing smaller inside an atom than quarks. Quarks are the smallest particles that are point-like which are the constituent particles of neutrons and protons. Neutrinos, quarks and electrons (and leptons in general) are all point-masses.
However, there's another theory. According to the string theory, the smallest fundamental particle is a string. This string that we are talking about is not a cord stretched across musical instrument okay? It is a hypothetical one-dimensional entity or object. It is a fundamental component of matter. A string has length equal to Planck Length, which is also the smallest possible length.
Planck Length is the smallest size observable in our universe.