  We calculate dust spectral energy distributions (SEDs) for a range of grain
sizes and compositions, using physical properties appropriate for five pulsar
wind nebulae (PWNe) from which dust emission associated with the ejecta has
been detected. By fitting the observed dust SED with our models, with the
number of grains of different sizes as the free parameters, we are able to
determine the grain size distribution and total dust mass in each PWN. We find
that all five PWNe require large ($\ge 0.1 \, {\rm \mu m}$) grains to make up
the majority of the dust mass, with strong evidence for the presence of
micron-sized or larger grains. Only two PWNe contain non-negligible quantities
of small ($<0.01 \, {\rm \mu m}$) grains. The size distributions are generally
well-represented by broken power laws, although our uncertainties are too large
to rule out alternative shapes. We find a total dust mass of $0.02-0.28 \, {\rm
M}_\odot$ for the Crab Nebula, depending on the composition and distance from
the synchrotron source, in agreement with recent estimates. For three objects
in our sample, the PWN synchrotron luminosity is insufficient to power the
observed dust emission, and additional collisional heating is required, either
from warm, dense gas as found in the Crab Nebula, or higher temperature shocked
material. For G$54.1$+$0.3$, the dust is heated by nearby OB stars rather than
the PWN. Inferred dust masses vary significantly depending on the details of
the assumed heating mechanism, but in all cases large mass fractions of
micron-sized grains are required.
