This study aimed to assess the impact of the color and font size of a dialogue box on an online physician–patient interaction page on patients’ perceptions of the physician’s authority and their willingness to schedule an offline appointment. A 2 × 2 between-group experiment was conducted to compare the effects of two dialogue box colors (gold vs. grey) and two font sizes (large vs. regular) on patients’ perceptions. The results showed that a larger font size had a significant positive impact on patients’ perceptions of the physician’s authority, and the use of a gold-colored dialogue box also had a significant positive effect. A significant interaction was found between the dialogue box color and font size and patients’ perceptions of the physician’s authority. In addition, it was found that positive perceptions of the physician’s authority significantly affected patients’ willingness to schedule offline appointments and played a fully mediating role in the path of page design affecting offline appointment intentions. This study provides evidence that the design elements of a dialogue box—particularly, its color and font size—can influence patients’ perceptions of a physician’s authority and their willingness to schedule an offline appointment. These findings suggest that modifying the page design could improve the effectiveness of physician–patient communication.