  We consider scalar perturbations of energy--density for a class of
cosmological models where an early phase of accelerated expansion evolves,
without any fine--tuning for graceful exit, towards the standard Friedman eras
of observed universe. The geometric procedure which generates such models
agrees with results for string cosmology since it works if dynamics is
dominated by a primordial fluid of extended massive objects. The main result is
that characteristic scales of cosmological interest, connected with the
extension of such early objects, are selected.}
