Could there be something smaller than quanta? Sure. Reality shrank beyond their senses and no doubt could go smaller still, until it was smaller than their ability to detect by any means whatsoever. Same in the other direction, with things big beyond the visible universe; for all they knew, their universe could extend forever, or be a neutrino in some larger universe. They could only see what they could see. Beyond that, the unknown. The unknowable.
