Size effects on metabolic rate include the effects of body size (biological) and sample size (methodological) and are to be found in cell, tissue, and body calorimetry. The biological size effect, known as Kleiber’s rule and consisting of a metabolic reduction with increasing body mass, is demonstrated by the size relationship of blood heat output in several mammalian species (cellular level) and by the differing metabolic behaviour of cardioplegic rat and dog heart samples (tissue level). The methodological size effect, known as the crowding effect and consisting of a metabolic decrease with increasing sample size, is demonstrated in human renal carcinoma cells (cellular level) and ischemic rat liver samples (tissue level) and is explained by a simple mathematical simulation. With respect to body calorimetry, it is stressed that metabolic size allometry may be temporarily inactivated in special biological adaptations (neonatal period, mammalian hibernation), and weight correction of metabolic rates may produce methodological problems when mass differences are mainly due to varying body fat content (circannual rhythms). In conclusion, careful size standardization is a prerequisite of comparability in biomedical calorimetry.