The present study of structural and physiological changes during the development of the cushion moss, Grimmia pulvinata, quantifies the size-dependence of various parameters of water relations such as changes in surface: volume ratio (S/V) or water loss rates, and also measures net CO2 gas exchange in the light and the dark. Larger cushions had lower S/V values than smaller ones and featured lower rates of area-based evapotranspiration, owing to higher boundary-layer resistance, but did not differ in relative water storage capacity (expressed as a percentage of d. wt). In combination, this leads to considerably longer hydration periods in larger cushions. By contrast, CO2 gas-exchange parameters were negatively correlated with size : larger cushions showed significantly lower (mass-based) rates of net photosynthesis and dark respiration. Using these data, we estimated carbon budgets during a drying cycle as a function of cushion size. When including alternations of dark and light periods, the relationship proved to be rather complicated. Depending on the time of hydration, net carbon budgets not only varied quantitatively with size but sometimes took on both positive and negative values depending on cushion size. We conclude that neglecting plant size can lead to unrepeatable or even misleading results in comparative ecophysiological studies, and therefore urge for adequate attention to be paid to size in these studies.