I found the answer to my problem !

I just put :

html {
zoom: 0.7;
}

in my responsif CSS code and my website size is now exactly perfect
Thank’s Chris for your help. Problem done 😀

Don’t worry its my english… so bad to explain a simple problem hahaha…

By default my website size is configured like that :
http://www.web-reggaeton.com/wp-content/uploads/2015/03/Capture-d’écran-2015-03-08-à-19.15.07.png
Everybody who come’s to see a news or my homepage see that.

BUT

I changed myself on my own navigator (Chrome, safari or mozilla) the resolution using “ZOOM” at 75% instead of 100% as before by default for every one who came visit my blog.

Using 75% zoom :
http://www.web-reggaeton.com/wp-content/uploads/2015/03/Capture-d’écran-2015-03-08-à-19.14.47.png

Do you think its possible to defined by default for everybody in my css a 75% zoom ?