The aim of this study was to assess the accuracy of digital mammography (DM), digital breast tomosynthesis (DBT) and synthetic mammography (SM) in tumor size measurements compared with histological tumor sizes. 71 breast cancer patients who underwent DM and DBT acquisitions simultaneously were included in this study. One radiologist with 8 years of experience in breast imaging measured tumor sizes independently in three separate sessions which include DM, DBT and SM images, respectively. The correlations between the measured tumor sizes on each imaging technique and histological sizes were analyzed using Spearman correlation test. The patients were categorized into two subgroups according to assigned breast density categories (dense and non-dense), and histological tumor sizes (≤2 cm and > 2 cm). To assess the agreement levels between the measured tumor sizes and histological sizes Bland-Altman analyses were performed for each imaging technique. The mean of histological size of tumors was 23.85 ± 16.57 mm (median: 20). The means of measured tumor sizes were 21.21 ± 13.59 mm (median: 19), 21.52 ± 13.42 mm (median: 19) and 18.97 ± 11.21 mm (median: 17) in DM, DBT and SM, respectively. The Spearman correlation values with histologic sizes were 0.814 (P < 0.001), 0.887 (P < 0.001), and 0.852 (P < 0.001) for DM, DBT and SM, respectively. In subgroup analyses, the correlation values showed decrement for tumors >2 cm in size compared to tumors ≤2 cm in size. DBT provides the most accurate tumor size measurements among mammographic imaging techniques and if mammography will be used in tumor size measurements, DBT should be preferred. •DBT provides the most accurate tumor size measurements.•DBT demonstrated the highest correlation with histological tumor sizes.•The correlation values show decrement for tumors above 2 cm in size.•Breast density did not affect the accuracy of tumor size measurements.