[untilted]

Just to add to this -- In QM/QFT there is an inverse relationship between energy & distance, meaning small distances (or sizes) correspond to high energy interactions (see e.g. [1]). One consequence is that at small enough scale (the Planck scale), the energy scale gets so large that quantum gravity effects are expected to be non-negligible. Formulating a theory of quantum gravity that fits into the Standard Model of particle physics & agrees with general relativity is an open problem in physics, therefore the Planck scale is at least the smallest distance that can conceivably be modeled given our current knowledge.

[1] https://physics.stackexchange.com/questions/731971/equivalen...