The official Youtube app for 3DS that was previously announced in a Nintendo Direct has finally launched for North America and Europe. The download will take up 113 blocks on your memory card. There is also the option to watch videos in High Quality and sets it to full screen instead of having a black box outside the video.
[Update: December 2, 2013] After using it for several days and reports from social media, the video quality currently available on the app, even on it’s “High Quality” mode is garbage and is not worth the time and space to download it at the current time.
(Source: My Nintendo News)