What is the smallest existing thing in theory and law? What is the smallest existing thing in theory and law?
 A: 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.
A: 
"What is the smallest existing thing in theory and law?"

The Merriam Webster Dictionary defines a "thing" as:


*

*: an object or entity not precisely designated or capable of being
designated

*a: an inanimate object distinguished from a living being 
b: a separate and distinct individual quality, fact, idea, or usually
entity 
c: the concrete entity as distinguished from

*...


A Photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force (even when static via virtual particles). Mass: 0 < 1×10−18 eV/c^2.
The photon has zero rest mass and always moves at the speed of light within a vacuum. Since the Photon is a Point Particle and has a size of zero you might say it's not a thing, nothing; that leaves us with:
The smallest real thing is the Neutrino. Mass: ≤ 0.120 eV/c^2.
The smallest theoretical thing is the Planck Particle. Radius: 5.72947×10−35 m, Mass: 3.85763×10−8 kg.
A: 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)...
