What is the smallest existing thing in theory and law?
The short answer to this question is that there is no answer because the question makes invalid (classical) assumptions. "Things" start to get blurry. They stop having a definite position, size, and boundary.
Take an electron for example. The electric field extends to infinity and the mass appears, to the best we can measure, to be a point in the center.
Theoretically, the Planck Length may be the smallest length that has any sort of physical meaning.
If by 'thing' you refer to a physical thing, not a distance (as opposed to what Brandon was thinking of when he mentioned the Planck Length), then I guess the answer is the Preon, which is believed to be the particle that Quarks/Leptons are made of.
But of course, maybe tomorrow some physicist discovers a smaller particle (back when I was in high school, the proton was still considered the smallest)...