Background/Aims: Colorectal polyps are removed to prevent progression to colorectal cancer. Polyp size is an important factor for risk stratification of malignant transformation. Endoscopic size estimation correlates poorly with pathological reports and several factors have been suggested to influence size estimation. We aimed to gain insight into the factors influencing endoscopic polyp size estimation. Methods: Images of polyps in an artificial model were obtained at 1, 3, and 5 cm from the colonoscope’s tip. Participants were asked to estimate the diameter and volume of each polyp. Results: Fifteen endoscopists from three large-volume centers participated in this study. With an intraclass correlation coefficient of 0.66 (95% confidence interval [CI], 0.62–0.71) for diameter and 0.56 (95% CI, 0.50–0.62) for volume. Polyp size estimated at 3 cm from the colonoscope’s tip yielded the best results. A lower distance between the tip and the polyp was associated with a larger estimated polyp size. Conclusions Correct endoscopic estimation of polyp size remains challenging. This finding can affect size estimation skills and future training programs for endoscopists.