I used to use this free one for windows. Can't find a similar SW for iP5... Just wanna enjoy HD video on the larger screen but even the YouTube app doesnt fit the entire screen.
IIRC, and this is on the Windows version, there is a selection box nearby with options like "strict", "anamorphic", etc. This selection will control whether you can enter your own width and height resolution. Keep in mind, you probably want to maintain the width/height ratio of the original.