Subatomic particles, such as quarks and electrons, are considered point-like particles in the Standard Model of particle physics. This means they are treated as having no size or spatial extent. According to this theoretical framework, they are considered to be fundamental particles with no internal structure.
However, it's important to note that the concept of "size" becomes more ambiguous at the subatomic level due to the inherent uncertainty associated with quantum mechanics. In quantum mechanics, particles are described by wave functions that assign a probability distribution to their position. These wave functions can spread out over space, implying a certain level of fuzziness or uncertainty in the particle's location.
In experimental measurements, the size of a particle can be probed indirectly by scattering experiments, where the interaction of the particle with other particles or fields provides information about its spatial distribution. For example, the charge distribution of a proton or neutron can be probed using techniques like electron scattering experiments. These experiments suggest that protons and neutrons have a finite size, with a radius on the order of femtometers (10^(-15) meters).
However, it's important to distinguish between the size of composite particles like protons and neutrons, which are made up of quarks and gluons, and the fundamental particles themselves. Fundamental particles, as far as our current understanding goes, are considered to be point-like with no spatial extent. The concept of their occupation of space is more appropriately described by their wave functions and associated probabilities.