The body size response to temperature is one of the most recognizable but still poorly understood ecological phenomena. Other covarying environmental factors are frequently invoked as either affecting the strength of that response or even driving this pattern. We tested the body size response in five species representing the Brachionus plicatilis cryptic species complex, inhabiting 10 brackish ponds with different environmental characteristics. Principal Component Analysis selected salinity and oxygen concentration as the most important factors, while temperature and pH were less influential in explaining variation of limnological parameters. Path analysis showed a positive interclonal effect of pH on body size. At the interspecific level, the size response was species- and factor-dependent. Under the lack of a natural thermo-oxygenic relationship, the negative response of size to temperature, expected according to 'size-to-temperature response' rules, disappeared, but a positive response of size to oxygen, expected according to predictions selecting oxygen as a factor actually driving these rules, remained. Our results confirm the crucial role of oxygen in determining the size-to-temperature patterns observed in the field.