  [Abridged] In this paper we explore grain size effects on wind properties of
carbon stars, using a generalized description of radiative cross sections valid
for particles of arbitrary sizes. The purpose of the study is to investigate
under which circumstances the small particle limit (SPL) may give acceptable
results, and to quantify the possible errors that may occur when it does not
hold. The time-dependent description of grain growth in our detailed
radiation-hydrodynamical models gives information about dust particle radii in
every layer at every instant of time. These grain radii are used for computing
opacities and determining the radiative acceleration of the dust-gas mixture.
It is shown that in the critical cases the effect of the generalized
description of dust opacities can be significant, resulting in more intense
mass loss and higher wind velocities compared to models using SPL opacities.
For well-developed winds, however, grain size effects on mass loss rates and
wind velocities are found to be small. Both groups of models tend towards lower
degrees of dust condensation compared to corresponding SPL models, due to a
self-regulating feedback between grain growth and radiative acceleration.
Consequently, the "dust-loss rates" are lower in the models with the
generalized treatment of grain opacities. We conclude that our previous results
on mass loss rates obtained with SPL opacities are reliable within a wide
region of stellar parameter space, except for critical cases close to
thresholds of dust-driven outflows where SPL models will tend to underestimate
the mass loss rates and wind velocities.
