The density fluctuations of nuclear matter are studied within a mean-field
model in which fluctuations are generated by an external stochastic field.
The constraints imposed on the random force by the fluctuation-dissipation
theorem are analyzed. It is shown that in the proximity of the borders of t
he spinodal region the assumption of a white-noise stochastic field can be
reliably used. The domain distribution of the liquid phase in the spinodal
decomposition of nuclear matter is derived. The related distribution of fra
gment sizes compares favorably with the experimental fragment distribution
observed in heavy ion collisions.