When it is said that a point particle has no size, it means that it is considered to be a mathematical point without any spatial extent. This notion is a fundamental assumption in certain theoretical models, such as classical point particles or elementary particles in quantum field theory.
In classical physics, point particles are treated as objects with zero size or dimension. They are conceptualized as mathematical entities with mass, charge, and other properties but without any physical extent. This assumption simplifies the mathematical descriptions of their motion and interactions.
In quantum field theory, elementary particles are described as excitations of underlying fields. These particles are considered to be point-like, meaning they have no spatial extension at the fundamental level. They are treated as localized in space but without any measurable size.
It is important to note that the concept of a point particle is an idealization and a simplification for mathematical convenience. In reality, all particles, even fundamental ones, are subject to quantum uncertainties and have a wave-like nature. At very small scales, such as the Planck scale, the concept of point-like particles may break down, and a more complete theory of quantum gravity might be required.
So, it is not merely that point particles are too small to be measured; rather, they are treated as having no spatial extent in the theoretical models used to describe them.