  There are two length-scales present simultaneously in all the principal
directions for three-dimensionally expanding, finite systems. These are
discussed in detail for the case of a longitudinally expanding system with a
transverse flow and a transverse temperature profile. For systems with large
geometrical sizes we find an $m_t$-scaling for the parameters of the
Bose-Einstein correlation function, which may be valid in the whole transverse
mass region for certain model parameters. In this limit, the Bose-Einstein
correlations view only a small part of the source. The large geometrical sizes
can be inferred from a simultaneous analysis of the invariant momentum
distribution and the Bose-Einstein correlation function. A preliminary analysis
of the NA44 data indicates that instead of a small fireball we are observing a
big and expanding snowball in $S + Pb $ reactions at CERN SPS energies.
