Size dependent strength of nanostructures

Dear all,

In recent years, it was suggested that the strength of nanoscale
structures increases with speciman size as a power-law. This has been demonstrated
many times for pillars, both experimentally and in discrete dislocation
dynamics simulations. Various values for the power-law exponent in pillars were
obtained, and most of them are about -0.6. However, I did not manage to find
any theory which explains both this typical behavior and the values of the exponent.
Is it a phenomenological law? If not, can anyone refer me a detailed model?

Thank you in advance,

Dan