  The Richardson exact solution for the reduced BCS Hamiltonian is applied to
examine how sensitive are the pairing characteristics (condensation energy,
spectroscopic gap, parity gap) to a specific configuration of single-electron
energy levels in nanosize metallic grains. Using single-electron energy spectra
in parallelepiped-shaped potential boxes with various volumes and aspect ratios
as a model of energy levels in grains, we show that this sensitivity is
extremely high. Just due to such an extreme sensitivity, the detailed shape of
grains cannot be detected through the pairing characteristics, averaged over an
ensemble of grains, even in the case of relatively small size dispersion within
this ensemble. We analyse the effect of the pairing interaction on the
excited-level spacings in superconducting grains and comment on the influence
of shape-dependent fluctuations in single-electron energy spectra on the
possibility to reveal this effect through tunnelling measurements.
