Viewers desire to watch video content with subtitles in various font sizes according to their viewing environment and personal preferences. Unfortunately, because a chunk of the subtitle—a segment of the text corpus displayed on the screen at once—is typically constructed based on one specific font size, text truncation or awkward line breaks can occur when different font sizes are utilized. While existing methods address this problem by reconstructing subtitle chunks based on maximum character counts, they overlook synchronization of the subtitle with the content, often causing the occurrence of misaligned text. We introduce OptiSub, a fully automated system that optimizes subtitle segmentation to fit any font size while ensuring synchronization with the content. Our system leverages the timing of speech pauses within the video for synchronization. Experimental results, including a user study comparing OptiSub with previous methods, demonstrate its effectiveness and practicality across diverse font sizes and input videos.