What is the smallest length scale ever measured? And, by the way, what is, or are, the measured values?
 A: 
Observation of a single electron in a Penning trap shows the upper limit of the particle's radius is 10^−22 meters

Reference.
A: The smallest length that has been directly measured is about $10^{-18}$m, and the measurement was done at the Large Hadron Collider. In a collider the length scale of the phenomena you are studying is related to the energy of your collision by:
$$ \Delta x \approx \frac{h c}{E} $$
where $h$ is Planck's constant, $c$ is the speed of light and $E$ is the collision energy. At the LHC the collision energy is around $10^{12}$eV - the total energy of the colliding protons is $7 \times 10^{12}$eV, but only a fraction of this energy is used in any particular quark-quark collision. If you feed these values into the equation above you get a value for $\Delta x$ of around $10^{-18}$m.
This isn't just playing with numbers. This is a real length measurement. For example you might ask if a quark is a fundamental particle or composite like protons. Well the measurements at the LHC tell us that if it's composite its size must be less than $10^{-18}$m or we would have seen evidence for its size at the LHC.
