This tweak has actually been implemented by Youtube itself in the form of Resize button. 
The tweak i am going will have the same effect (of resizing the video) as done by the “Change Player Size” button. This trick will be useful when viewing videos that have a set size of 320×240 (Regular resolution of videos on Youtube).
In order to view the larger resolution of videos just add the following snippet to the url of the youtube video :
&fmt=6
OR
&fmt=18
For example, the url of a video is : http://www.youtube.com/watch?v=eF-rxgaOeB4
Just add the above mentioned snippet to get : http://www.youtube.com/watch?v=eF-rxgaOeB4&fmt=6
Adding the above mentioned code to the end of url increases the video size to 480×360.
Before Adding The Code
After Entering The Code
