Methods of assessing the structure and function of food webs are needed to provide a basis for assessing large-scale direct (e.g. fisheries) and indirect (e.g. climate change) effects of human activities on marine ecosystems. We present a simple synthesis of the complex structure and function of a real marine food web, based on analyses of body size distributions, production-body size relationships and trophic level-body size relationships. We show how size-based estimates of production, species richness and trophic level (from nitrogen stable isotope analysis) can be used to quantify trophic transfer efficiency, mean predator-prey body-mass ratios and the mean ratio of the number of predator to prey species in marine food webs. We applied these methods to the central North Sea, and estimated transfer efficiencies of 3.7 to 12.4%, a mean predator-prey body-mass ratio of 109:1 and a mean ratio of the number of predator to prey species of 0.34. We conducted sensitivity analyses to show how differences in the fractionation of d15N and changes in the slope of the relationship between production and d15N affected our predictions. Our estimates of transfer efficiency and mean predator-prey body-mass ratios are similar to those obtained by costly and labour-intensive diet- and ecosystem-modelling studies. Coupled analyses of size and trophic structure may provide a method for validating ecosystem models and assessing human impacts on marine ecosystems.