Customizing WordPress Theme Twenty Eleven Automatic Width

First I don't know any about CSS.

Do not edit the Twenty Eleven theme. It is the default WordPress theme and having access to an unedited version of the theme is vital when dealing with a range of site issues.

For this reason, it is recommended that you consider creating a child theme for your customisations.

wget http://wordpress.org/extend/themes/download/twentyeleven.1.4.zip
unzip twentyeleven.1.4.zip
mv twentyeleven twentyeleven_child
cd twentyeleven_child/

Modify sytle.css

Change the Theme Name to a different name like : Twenty Eleven Child

/*
Theme Name: Twenty Eleven Child
Theme URI: http://wordpress.org/extend/themes/twentyeleven
Author: the WordPress team
Author URI: http://wordpress.org/

mv twentyeleven_child /www/wordpress/wp-content

Once your child theme is active, we should be able to help with the customisation.

To make your WordPress width fits any resolution Automatic, just common out the [max-width:] line.

/* =Structure
----------------------------------------------- */

body {
        padding: 0 2em;
}
#page {
        margin: 2em auto;
/*      max-width: 1000px;*/
}

Then active the child Theme, and you are done!