Oxygen availability limits the maximum potential size attainable in benthic gammaridean amphipods from the suborder to the generic level, while also influencing size within species. In this paper we investigate the effect of oxygen on 15 size spectra worldwide, established by compiling maximum length data of more than 2000 amphipod species. We used TS95/5 as a proxy for maximum size and defined this as the threshold size between the 95% smallest and the 5% largest species at any given site. Our data show that beside TS95/5, minimum, mean and modal sizes, as well as all 10% increment threshold sizes (from TS10/90 to TS90/10) also vary significantly with oxygen concentration. Size distributions are very similar in shape from one geographical area to another, whatever the width of the spectrum, hence if overall numbers of species stay constant more small species should coexist at low than at high latitudes. No amphipod species were found in water with an oxygen content lower than the minimum requirement predicted (175 mmol O2 kg−1). Our data also show that minimum adult amphipod size is probably limited by the minimum possible egg size, whereas maximum size is set by the physico-chemic ceiling of oxygen availability.