  The characteristic size of penumbral structures are still below the current
resolution limit of modern solar telescopes. Though we have seen a significant
progress in theoretical work over the last decades no tight constraints can be
placed on the size of penumbral structures in order to favor models with
relatively large and thick magnetic flux elements, just at or below the current
resolution limit, or on the other hand, clusters of optically thin
micro-structures.
  Based on a macroscopic 2-component inversion and the approach of polarized
radiative transfer in stochastic media, we have estimated the characteristic
length scale of the magnetic fluctuation in a sunspot penumbra from observed
Stokes spectra. The results yield a coherent picture for the entire magnetic
neutral line of the penumbra and indicate that the magnetic fluctuations have a
typical length scale between 30 km and 70 km.
