The size distribution of frazil ice is currently unconstrained in ice shelf cavity modeling. Here we observe the time-dependent behavior of the number and size of frazil ice particles in an Ice Shelf Water plume. A novel acoustic scattering inversion was used to infer frazil ice crystal diameters, assuming a log-normal distribution. Observation sites were on land-fast sea ice approximately 13 and 33 km from the front of the McMurdo Ice Shelf, Antarctica. The water column from the ice-water interface to 30 m below mean sea level was monitored over 3 weeks in November of 2016 and 2017. At 15 m below sea level the mean frazil crystal diameter was $\sim$\SI{1}{\milli\metre}. Fractional ice volume, derived from frazil crystal size and number density, correlates with in-situ supercooling (up to \SI{50}{\milli\kelvin} at \SI{15}{\metre} below sea level). The data presented here provide valuable input for model initiation and evaluation.