  In this work, the well known power-law relation between strength and sample
size, $d^{-n}$, is derived from the knowledge that a dislocation network
exhibits scale-free behaviour and the extreme value statistical properties of
an arbitrary distribution of critical stresses. This approach yields
$n=(\tau+1)/(\alpha+1)$, where $\alpha$ reflects the leading order algebraic
exponent of the low stress regime of the critical stress distribution and
$\tau$ is the scaling exponent for intermittent plastic strain activity. This
quite general derivation supports the experimental observation that the size
effect paradigm is applicable to a wide range of materials, differing in
crystal structure, internal microstructure and external sample geometry.
