  The Pippard coherence length $\xi_0$ (the size of a Cooper pair) in an
extreme type-I superconductor was determined directly through high-resolution
measurement of the nonlocal electrodynamic effect combining low-energy muon
spin rotation spectroscopy and polarized neutron reflectometry. The
renormalization factor $Z$=m_cp*/2m (m_cp* and m are the mass of the Cooper
pair and the electron, respectively) resulting from the electron-phonon
interaction, and the temperature dependent London penetration depth
$\lambda_L(T)$ were determined as well. A general expression linking $\xi_0$,
$Z$ and $\lambda_L(0)$ is introduced and experimentally verified. This
expression allows one to determine experimentally the Pippard coherence length
in \textit{any} superconductor, independent of whether the electrodynamics is
local or nonlocal, conventional or unconventional.
