Predation is an important factor that influences the behavior of prey species. When approached by a predator, individuals must make escape decisions that may be sensitive to the level of perceived risk. Both predator size and prey group size influence vulnerability and may influence this assessment. If fish perceive larger predators as a greater threat, differences in predator size should reflect differences in their antipredator response. Group size may also influence antipredator response in congregated fish. We investigated the flight initiation distance (FID) of differently sized groups of dusky damselfish (Stegastes nigricans) to measure their antipredator response to varying sizes of artificial stimuli. We found no effect of shoal size on FID, indicating that damselfish risk assessment is not influenced by group size. However, we did find that stimulus size interacted with starting distance (SD) to affect FID, with damselfish distinguishing between stimulus sizes at shorter SD but not at longer SD. Adjustment of FID in response to varying stimuli indicates that damselfish risk assessment is plastic, and consequently, antipredator responses may be adaptable in changing ocean ecosystems. Because damselfish are herbivores that feed on algae in coral reefs, understanding factors affecting their assessment of risk is important to understanding the overall ecosystem structure. These behavioral responses could be significant in the context of anthropogenic activities, mainly overfishing, which shifts community composition by disproportionately removing larger top predators.