Objective To evaluate the clinical importance of endometrial polyp size measured using saline infusion sonohysterography (SIS) before performing a hysteroscopic resection in predicting premalignant/malignant lesions. Methods A retrospective observational study analysis was conducted of 365 patients, who underwent SIS, in a reference hospital. The longest plane of the polyp size was taken as base. Polyps were classified as benign, premalignant, or malignant. Results The rates of premalignant and malignant lesions were 7.4% and 0.9%, respectively. The mean polyp size was 17.7 +/- 0.5 mm in benign patients and 23.7 +/- 1.8 mm in premalignant/malignant individuals (P < 0.001). In the group of polyps that were 0-10, 10-20, 20-30, and >30 mm, premalignancy/malignancy rates were 0.0%, 4.8%, 13.3%, and 18.8%, respectively. The cut-off value for polyp size to be able to predict lesions was calculated as 22.5 mm (sensitivity: 63%, specificity: 80%) on receiver operating characteristics curve analysis (P = 0.001, area under the curve 0.732). The power of the study was calculated as 90.86%. Conclusion During the female reproductive years, endometrial polyps smaller than 10 mm, as measured in SIS, can be followed. However, when the polyp size is 22.5 mm or more, especially in postmenopausal women, treatment should be planned.