  The question of the exact nature of the phase transition in two-flavor QCD is
still under discussion. Recent results for small quark masses in simulations
with 2+1 flavors show scaling behavior consistent with the O(4) or O(2)
universality class. For a precise determination, an assessment of deviations
from the ideal scaling behavior due to finite quark masses and finite
simulation volumes is necessary.
  We study the scaling behavior at the chiral phase transition with an
effective quark-meson model. In our Renormalization Group approach, the quark
masses in the model can be varied from the chiral limit over a wide range of
values, which allows us to estimate scaling deviations due to large quark
masses and the extent of the scaling region. We conclude that scaling
deviations are already large at pion masses of 75 MeV, but that the effect is
difficult to see in the absence of results for even smaller masses. Comparing
results only in a narrow window of pion masses leads to the observation of
apparent scaling behavior. While the scaling deviations are not necessarily
universal, we expect that this may affect current lattice simulation results.
  By placing the system in a finite box, we investigate the transition between
infinite-volume scaling behavior and finite-size scaling. We estimate that
finite-size scaling behavior can be tested in regions where pion mass times box
size is approximately 2 - 3, which is smaller than in most current lattice
simulations. We expect that finite-volume effects are small for pion masses of
75 MeV and lattice aspect ratios with TL > 8, but that they will become
significant when pion masses in lattice simulations become smaller.
