The nicovideo-player-expander is a Chrome add-on designed to enhance the viewing experience on NicoNico Douga by providing additional player size options. Users can switch between two methods of expanding the player: 'Theater Mode' and 'Browser Maximize'. These options can be accessed easily through an icon located at the bottom right of the player or by using keyboard shortcuts. The add-on addresses the limitation of the NicoNico player, which only offers full-screen viewing, by introducing these new functionalities.

This free extension is particularly useful for users looking for more flexibility in viewing content on NicoNico Douga. The shortcuts allow for quick toggling between modes, enhancing user convenience. The extension is a valuable tool for those who want to customize their video-watching experience, making it more enjoyable and user-friendly.