The 2019/2020 Australian New Year (ANY) pyrocumulonimbus (pyroCb) events injected massive quantities of smoke into the stratosphere, significantly altering Earth's radiative balance and chemical composition. Accurately simulating these impacts in Earth System Models (ESMs) remains challenging, partly because standard bulk aerosol schemes assume particle properties based on tropospheric smoke. In this study, we leverage the NASA GEOS model coupled with the CARMA sectional microphysics module to simulate the rapid microphysical evolution of the ANY pyroCb plume. Our sectional aerosol model simulations reveal that extreme initial number densities drive rapid coagulation, producing stabilized aged particles with an effective radius (<em>R_eff</em>) of ~ 0.30 &micro;m, consistent with in-situ and lidar observations. Based on these results and offline Mie-theory calculations, we derive an observationally constrained configuration (<em>R</em>_<em>eff</em> = 0.30 &micro;m, &sigma; = 1.2, with moderated near-UV absorption) for the computationally efficient GOCART-2G bulk aerosol module. Compared to the finer-mode baseline, the updated configuration significantly improves simulated aerosol extinction and &Aring;ngstr&ouml;m Exponent, bringing the plume's spatial distribution and longevity into closer agreement with SAGE-III/ISS and OMPS-LP satellite records. Furthermore, pairing the larger particle size with observationally constrained absorption moderates excessive shortwave radiative heating, enabling more realistic large-scale transport. The larger particle size assumption also reduces the available aerosol Surface Area Density (SAD) by more than a factor of three. Our findings highlight a critical sensitivity to particle size assumptions in simulating large pyroCb events, demonstrating that constraining this property is essential for accurately assessing the plume's radiative impact and its role in heterogeneous chemistry.