Introduction: An attractive smile helps people to feel more self-confident and look younger and more attractive. Smile evaluation and smile design are therefore important aspects of restorative and orthodontic diagnosis and treatment planning. Aim: The purpose of this study was to determine the influence of tooth size on smile attractiveness. Materials and methods: The study was conducted in 2017 in Sanandaj, Iran. The participants were selected by convenience sampling among 50 dental students, 50 art students, and 50 laypeople. A color photograph of a posed smile was chosen from the internet. The maxillary anterior dentition was digitally altered to produce different tooth sizes with width-to-height ratios of 65%, 70%, 75% (original photograph), 80%, 85%, and 90%. Six images of each subject were paired into 16 possible combinations and presented to three groups (dental students, art students, and laypeople). The Wilcoxon signed-rank test was used for data analysis. Results: Statistical analysis showed that the width-to-height ratios of 65% and 70% for the maxillary anterior teeth was considered least attractive by the participants in all three groups. Conclusion: Minimal tooth size should be taken into account during restorative treatment planning, and excessive tooth proportions should be considered esthetically problematic.