MP4 resolution help

[iiSupaSaiyanii]

Ok so I am currently tring to download an MP4 video onto my PSP and the max resolution it can have is 720X480. The video I have is 854X480 but I was told that you can permanently change the size of the video for download on Mplayer but I am currently having trouble trying to find out how and also how do I save the video from Mplayer to a different location. Help anyone?