  We systematically address superconductivity of Pb nano-islands with different
thicknesses and lateral sizes via a scanning tunneling microscopy/spectroscopy.
Reduction of the superconducting gap is observed even when the island is larger
than the bulk coherence length and becomes very fast below ~ 50 nm lateral
size. The suppression of gap with size depends to a good approximation only on
the volume of the island and is independent of its shape. Theoretical analysis
indicates that the universal quenching behavior is primarily manifested by the
mean number of electronic orbitals within the pairing energy window.
