Dust grain-size distribution, n(a), showing the posterior probability distribution of fits to the GRB 221009A X-ray rings, with a model based on the MRN (Mathis et al. 1977) distribution with an additional large-grain size lognormal component. The proposed grain-size distribution of Hensley & Draine (2023) is shown for comparison, and the best-fit parameters of this parametrisation is shown as well. The dotted line indicates the 95th percentile (2σ) upper envelope on the grain-size distribution. Strong limits can be placed on the population of a ~ 0.4 μm grains, which must be several orders of magnitude less abundant than grains with a ~ 0.2 μm. Because of the lack of grains at this size, the Hensley & Draine (2023) grain-size distribution is disfavoured for this sight line. Larger grains (≳1 μm) are not strongly constrained as these predominantly scatter at smaller angles than we measure with these data.