ObjectivesTo investigate how studies determine the sample size when developing radiomics prediction models for binary outcomes, and whether the sample size meets the estimates obtained by using established criteria.MethodsWe identified radiomics studies that were published from 01 January 2023 to 31 December 2023 in seven leading peer-reviewed radiological journals. We reviewed the sample size justification methods, and actual sample size used. We calculated and compared the actual sample size used to the estimates obtained by using three established criteria proposed by Riley et al. We investigated which characteristics factors were associated with the sufficient sample size that meets the estimates obtained by using established criteria proposed by Riley et al.ResultsWe included 116 studies. Eleven out of one hundred sixteen studies justified the sample size, in which 6/11 performed a priori sample size calculation. The median (first and third quartile, Q1, Q3) of the total sample size is 223 (130, 463), and those of sample size for training are 150 (90, 288). The median (Q1, Q3) difference between total sample size and minimum sample size according to established criteria are -100 (-216, 183), and those differences between total sample size and a more restrictive approach based on established criteria are -268 (-427, -157). The presence of external testing and the specialty of the topic were associated with sufficient sample size.ConclusionRadiomics studies are often designed without sample size justification, whose sample size may be too small to avoid overfitting. Sample size justification is encouraged when developing a radiomics model.Key PointsQuestionSample size justification is critical to help minimize overfitting in developing a radiomics model, but is overlooked and underpowered in radiomics research.FindingsFew of the radiomics models justified, calculated, or reported their sample size, and most of them did not meet the recent formal sample size criteria.Clinical relevanceRadiomics models are often designed without sample size justification. Consequently, many models are too small to avoid overfitting. It should be encouraged to justify, perform, and report the considerations on sample size when developing radiomics models.Key PointsQuestionSample size justification is critical to help minimize overfitting in developing a radiomics model, but is overlooked and underpowered in radiomics research.FindingsFew of the radiomics models justified, calculated, or reported their sample size, and most of them did not meet the recent formal sample size criteria.Clinical relevanceRadiomics models are often designed without sample size justification. Consequently, many models are too small to avoid overfitting. It should be encouraged to justify, perform, and report the considerations on sample size when developing radiomics models.Key PointsQuestionSample size justification is critical to help minimize overfitting in developing a radiomics model, but is overlooked and underpowered in radiomics research.FindingsFew of the radiomics models justified, calculated, or reported their sample size, and most of them did not meet the recent formal sample size criteria.Clinical relevanceRadiomics models are often designed without sample size justification. Consequently, many models are too small to avoid overfitting. It should be encouraged to justify, perform, and report the considerations on sample size when developing radiomics models.