Eyewitness identification via lineup procedures is an important and widely used source of evidence in criminal cases. However, the scientific literature provides inconsistent guidance on a very basic feature of lineup procedure: lineup size. In two experiments, we examined whether the number of fillers affects diagnostic accuracy in a lineup, as assessed with receiver-operating characteristic (ROC) analysis. Showups (identification procedures with one face) led to lower discriminability than simultaneous lineups. However, in neither experiment did the number of fillers in a lineup affect discriminability. We also evaluated competing models of decision-making from lineups. This analysis indicated that the standard Independent Observations (IO) model, which assumes a decision rule based on the comparison of memory strength signals generated by each face in a lineup, is incapable of reproducing the lower level of performance evident in showups. We could not adjudicate between the Ensemble model, which assumes a decision rule based on the comparison of the strength of each face with the mean strength across the lineup, and a newly introduced Dependent Observations model, which adopts the same decision rule as the IO model, but with correlated signals across faces. We draw lessons for users of lineup procedures and for basic research on eyewitness decision making. (PsycInfo Database Record (c) 2020 APA, all rights reserved).