Individual plant size often determines the vital rates of growth, survival, and reproduction. However, size can be measured in several ways (e.g., height, biomass, leaf length). There is no consensus on the best size metric for modelling vital rates in plants. Demographic datasets are expanding in geographic extent, leading to choices about how to represent size for the same species in multiple ecological contexts. If the choice of size variable varies among locations, inter-population comparative demography increases in complexity. Here, we present a framework to perform size metric selection in large-scale demographic studies. We highlight potential pitfalls and suggest methods applicable to diverse study organisms. We assessed the performance of five different size metrics for the perennial herb Plantago lanceolata across 55 populations on three continents within its native and non-native ranges, using the spatially replicated demographic dataset PlantPopNet. We compared the performance of each candidate size metric for four vital rates (growth, survival, flowering probability, and reproductive output) using generalized linear mixed models. We ranked the candidate size metrics based on their overall performance (highest generalized R²) and homogeneity of performance across populations (lowest total magnitude of, and variance in, population-level error). While all size variables performed well for modelling vital rates, the number of leaves (modelled as a discrete variable, without transformation) was selected as the best size metric, followed by leaf length. We show how to interrogate potential trade-offs between overall explanatory power and homogeneity of predictions across populations in any organism. Synthesis: Size is an important determinant of vital rates. Using a dataset of unprecedented spatial extent, we find a) consistent size-based models of growth, survival, and reproduction across native and non-native populations of this cosmopolitan plant species and b) that several tested size metrics perform similarly well. This is encouraging for large-scale demographic studies and for comparative projects using different size metrics, as they may be robust to this methodological difference.

PlantPopNet (www.plantpopnet.com) collaborators collect demographic information on 65 naturally occurring populations of P. lanceolata across three continents. The present study included 55 populations that had at least two consecutive yearly censuses, presented here. Each population consists of an initial 100 individuals marked in naturally occurring populations and re-visited yearly at the peak of the flowering season. New recruits within the original plots were recorded and followed in subsequent years. The number of rosettes, number of leaves per rosette, length of the longest leaf, and width of the longest leaf for each rosette, flowering status (flowered, not flowered), reproductive output, and survival or death of each individual were recorded at each annual census. For further information on the PlantPopNet protocol, see Buckley et al. (2019). This data is presented as it was used to perform a study on a subset of the plantpopnet data. For said study, we used the first transition (from the start of the study at that site to one year after, i.e., the first two years of census data) from each population for the study, which this data accompanies. We considered one genet to be one individual. Rosettes (ramets) linked by the same rooting system were added to the size of the overall genet. This is the level of precision of the data that is presented here. In addition, we estimated size using five candidate size metrics: number of leaves, estimated biomass, total leaf area, total leaf length, and length of the longest leaf for each individual in the dataset (see metadata and accompanying paper for details of the methodology.