You may need YTO which stands for YouTube Options. Here is what the app does,
YouTube Options: disable pre-roll ads, in-video ads, annotations, and auto-play; change resolution, display size, optional Flash pre-buffering, disable DASH, looping/replay, video audio volume, SSL, keyboard controls, use your mouse scroll-wheel to change the size, volume, or fast forward or fast rewind the video, auto-resize the video when scrolling down the page; download all available versions of the media (this will require installing the full version from the developers website).
Once you sync Chrome with your Google account you can add apps such as YTO. After installing you will be given various options to customize the app. This could include the display size of the video or if you want ads displayed.
Here is what YouTube will look like after its installed.