The video screen is small because you do not have enough internet bandwidth to see the full video. You must be using a broadband connection such as a cable or DSL modem. Right click on the playing video and select statistics. The bit rate value should be 300 kbps or higher. Make sure you are not downloading music, movies, or games at the same time as this consumes your bandwidth.
Note that if you are on a shared network, other people may be consuming your bandwidth. For example if your children are using the web at the same time you are. Contact your internet service provider if you suspect you are getting insufficient bandwidth.