How do you command Vidéo-Size with keyboard ?

I'm not so sure I understand your question. Are you talking about the ctrl+mouse wheel combo?
What application are you using to watch videos?

xine supports the double zoom i believe

Hey guys, where did you learn to read ???

I WANT THE SHORTCUT FOR DOUBLE-SIZE A VIDEO WHEN PLAYING. (like "2" in totem, or "CTRL+2" in Media Player Classic). Is is complicate to understand ? :-///////////////

and YES, i am using MPLAYER, why you think I ask an answer for MPLAYER then ??? >_____________<

MPlayer doesn't have a gui. Which GUI are you using: gmplayer, kplayer, kmplayer, emotion, etc.?

...

I am using gmplayer, but that doesn't matter, because all I want is a shorcut to double-size the Video Window...

I know I can use mouse to modify the window size, but I want a keyboard shortcut...

Please don't complain at the volunteers here who are trying to help you to achieve what you want.

There is no keyboard shortcut to do what you are asking. The reason why people are suggesting other media players is because the functionality to do what you want isn't there in mplayer but is in other players.

There is only the "f" key that I am aware of in mplayer which switches between windowed and full screen. I discovered this by googling for "mplayer keyboard shortcuts" and reading the man page for mplayer by typing "man mplayer" in a console.

If you are not happy with the keyboard shortcuts available in mplayer then by all means file a bug against the package:-

https://bugs.launchpad.net/ubuntu/+source/mplayer/+bugs