Sparse forest inventories often provide too few plots to estimate species-specific tree-size distributions for individual stands. We develop a general Bayesian magnitude-composition model that separates overall abundance from its allocation among categories. A negative-binomial model describes total counts, while multinomial models describe how those counts are allocated among categories. In forest inventory, this construction allocates trees among species and then size classes within species, without imposing a parametric form on the tree-size distribution. Covariates inform both abundance and composition. Shared latent factors represent residual spatial and group-level dependence among categories using relatively few processes, while smoothing priors on regression coefficients borrow information across neighboring size classes. Nearest-neighbor Gaussian processes support efficient spatial computation. Joint posterior prediction and area-weighted aggregation yield stand-tables---tree densities by species and size class---and derived measures such as size-class diversity, with associated uncertainty. Simulations show small estimation errors and near-nominal predictive-interval coverage for forest and stand summaries. An application combining field plots and light detection and ranging (LiDAR) covariates illustrates estimation of species-specific distributions of diameter at breast height and size-class diversity in sampled and unsampled stands.

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