The intensity and stability of thermoremanent magnetization (TRM) acquired by synthetic Fe7S8 particles in the size range 1--29 &mgr;m has been determined. The intensity falls with increasing particle size. The unblocking temperature increases with increasing particle size. The intensity-particle size dependence can be fitted to a theory, in which a fraction of the particles fails to nucleate a domain wall on cooling, only if the TRM of the resultant monodomain fraction is much smaller than that expected of crystals of this size. This could arise if the particles were made of nanocrystals of much smaller size. The data may be given a much simpler theoretical description by adopting a particle size dependent effective demagnetizing factor with a fully formed domain structure with highly mobile walls above the blocking temperature. ¿ 1998 American Geophysical Union