Cannibalism among fish is an interaction affecting both intra- and interspecific relationships in aquatic environments. It is related to the size and developmental stage, with prey size determined by morphometric measurements. In general, cannibalism only occurs when the predator's mouth width is greater than the prey's body depth. In this way, understanding these limitations during ontogenetic development is important, as it significantly influences the dynamics of cannibalism. This study aimed to evaluate the maximum size of conspecific prey that the larvae of matrinxã (Brycon amazonicus) are morphologically able to ingest. For this, the morphometric measurements of body depth (BD) and maximum mouth opening (MMO) in relation to the total length (TL) of the larvae were considered. BD and MMO showed negative allometric growth. The maximum prey size was expressed as a proportion of the size of the cannibal, which shows that cannibals can ingest prey from 92 to 106% of the TLcannibal. Therefore, the mathematical model of maximum cannibal-prey size for B. amazonicus shows rapid growth, possibly due to heterogeneous growth that, in turn, can increase the display of aggressive behavior, and it is not possible to determine a predatory morphological limitation. Studies with longer observation periods, focusing on predator-prey interaction and capture time, should be conducted to reduce cannibalism and increase survival and growth. Reducing cannibalism in B. amazonicus is essential for advancing the production of this economically and ecologically important species in the Amazon region.