We show that the largest tropical species of terrestrial poikilotherms from 25 taxa exceed, in linear body size, the largest representatives of the same taxa from the temperate (e.g. Great Britain) and polar (e.g. Wrangel Island) zones by 3.2 and 5.7 times, respectively. Here we develop a theoretical approach which quantitatively explains the observed body size patterns and relates them to ambient temperature under the assumption that there exists a temperature-independent critical minimum value of mass-specific metabolic rate bmin, a fall below which is incompatible with successful biological performance. This value sets an upper limit to linear body size within a taxon. Mass-specific metabolic rate decreases with increasing body size but, in poikilotherms, grows exponentially with ambient temperature. Such compensation of the size-related drop in mass-specific metabolic rate b by higher ambient temperature extends the permitted range of body sizes for which b≥bmin. As a result, the maximum linear body size grows approximately twofold per each ten degrees of increase in ambient temperature. We also discuss why this prediction does not apply to interspecific comparisons of aquatic poikilotherms, for which an opposite trend is to be expected. We quantify in theory the maximum body size patterns recently reported for benthic gammaridean amphipods.