What is the smallest observable structure in the universe? I've been wondering about the Planck length recently, but it is not observable. What is the smallest actually observable structure in the universe?
 A: 
What is the smallest actually observable structure in the universe?

The smallest structure that I have seen is the electron cloud around an atom.
A: It depends on what you mean by observable. The smallest confirmed particle is the electron neutrino. Observation/measurement is another discussion in itself.
A: Your question comes with an extra difficulty: the meaning of distances and sizes at the quantum level.
From the current understanding of particle physics, all particles are point-like and hence volumeless. Of course this is an interpretation of the theory and this statement would require the experiments to probe arbitrarily small distances, which does not happen.
Insofar the shortest distances probed are around $10^{-16}$ cm, while the LHC is expected to achieve $10^{-17}$ cm. But these distances do not immediately correspond to structures.
If we refer to smallest structures to volume occupying entities in nature, then the smallest known will have to be mesons (quark and anti-quark pair, like the pion $\pi$), whose volume and shape is still a matter of research, but you can take an upper bound to be of order of the size of the nucleus of Hydrogen (a proton): $10^{-12}$ cm.
References:
https://www.ias.edu/articles/large-hadron-collider
http://en.wikipedia.org/wiki/Orders_of_magnitude_(length)#1E-15
