Look for the function on your Wii. Widescreen will not be available on Wind Waker since it released before Widescreen settings became common for games.
Turn on your Wii. Go to Wii -> Wii Settings -> Screen -> Widescreen settings.
If that is already set up, it means your Wii is keeping Windwaker at a Fullscreen setting.
Your last option is to change the Widescreen settings through your TV.