Smallest Hypothetical Object in the Universe

Keeping in mind what we just said about the idea of size being irrelevant on a quantum level, there exists something known as string theory and although it is slightly controversial it proposes that all subatomic particles are composed of vibrating strings that interact in order to create things like mass and energy. So, although these strings would technically have no physical dimensions, according to our puny human ability to reason one could say that these strings would be in some sense the smallest objects in the universe.