If you want to improve the resolution of the video file, you have to encode it in your preferred resolution using for example Expression Encoder. This is a tutorial on how to encode a video file using Expression Encoder
On the other hand, if you just want to change the size of the video playing inside RadMediaPlayer, you can use the VideoWidth
properties. More info on how to use them, you can find here
Let us know how this works for you.
the Telerik team