A longstanding goal of ecology has been to determine the mechanistic processes that underly pervasive macroecological patterns. Community size distributions, in particular, have long been of interest. We present a zeroth order model for the structure and dynamics of plant communities under demograhic and resource steady state. The theory is an ecological extension of metabolic scaling theory. It shows how sizes of plants affect competition, resource use, growth and death, and how these individual-level processes determine the structure and dynamics of plant communities. It deliberately makes simplifying assumptions but, nevertheless, provides a quantitative, predictive framework for understanding the structure and dynamics of an average idealized plant community. In doing so the theory provides a basis by which additional secondary details and deviations due to site and taxa specific phenomena can be quantified and added. The model predicts how the density of individuals, turnover, and filling of space are determined by scaling rules which are invariant in space and time. It shows the inextricable linkage between size-dependent between size dependent growth and mortality of individuals. Supported by macroecological data from long-term forest dynamics plots across the globe, our results indicate that the size structure and the dynamics of plant communities are described by general scaling rules. The predictive nature of the theory is due to two fundamental biological phenomena: metabolism and allometry. This is in contrast to recent phenomenological models (that have incorrectly been presented as alternative models to metabolic scaling theory) where, for example, size distributions are calculated from empirical data on growth, mortality, and recruitment rather than from a unified, integrated mechanistic theory where all of these attributes can be derived from general principles. We discuss the implication of these sclaing rules for community ecology in general.