The anticipated widespread use of augmented reality (AR) head-mounted displays (HMDs) as daily mobile devices underscores the importance of button size, a critical factor influencing user interface effectiveness and task performance. While most mobile device interactions occur during walking, prior research on button touch has primarily been conducted in stationary VR or AR HMD environments, often neglecting the dynamic context of mobile use. This study addresses this gap by optimizing button sizes in AR user interfaces specifically for walking conditions. The objective is to identify an ideal button size that maximizes task performance accuracy while minimizing visual obstruction. Utilizing a randomized complete block design, participants interacted with buttons of various sizes (15–55 mm) in an AR setting. Findings indicate that larger buttons enhance task performance though excessively large buttons can cause visual obstruction and dizziness. The optimal size was identified to be 45 mm, balancing design and user experience. This research contributes to human-computer interaction in AR, emphasizing user-centered design in emerging technologies.