We study the magnetic-field-induced breakdown of superconductivity in
nm-scale metal grains having a mean electron level spacing d similar o
r equal to <(Delta)over tilde> (bulk gap). Using a generalized variati
onal BCS approach that yields good qualitative agreement with measured
spectra, we argue that Pauli paramagnetism dominates orbital diamagne
tism, as in the case of thin films in a parallel magnetic field. Howev
er, the first-order transition observed for the latter can be made con
tinuous by finite size effects. The mean-field procedure of describing
the system by a single pairing parameter Delta breaks down for d simi
lar or equal to <(Delta)over tilde>.