  We calculate universal finite size scaling functions for the order parameter
and the longitudinal susceptibility of the three-dimensional O(4) model. The
phase transition of this model is supposed to be in the same universality class
as the chiral transition of two-flavor QCD. The scaling functions serve as a
testing device for QCD simulations on small lattices, where, for example,
pseudocritical temperatures are difficult to determine. In addition, we have
improved the infinite volume limit parametrization of the scaling functions by
using newly generated high statistics data for the 3d O(4) model in the high
temperature region on an L=120 lattice.
