the neutrino is considered one of the smallest known particles with a finite size, but its precise size is still not precisely determined. Neutrinos are subatomic particles with extremely low mass and are known to interact only weakly with matter, making them difficult to detect and measure accurately.
The size of a particle is often associated with its wavelength, as described by the de Broglie wavelength. The de Broglie wavelength of a particle is inversely proportional to its momentum. Since neutrinos have very low mass and are relativistic (meaning they travel close to the speed of light), their de Broglie wavelengths are relatively long. This is why neutrinos are typically described as having very small or negligible sizes.
It's important to note that our understanding of particle physics is constantly evolving, and new discoveries can lead to updates in our knowledge. Future experiments and theoretical developments may shed more light on the precise nature and size of particles, including neutrinos, or may reveal the existence of particles that are even smaller or possess unique properties that challenge our current understanding.