don't edit Twenty Eleven directly;
either create a child theme for the customisations, or if you only want to edit the css, use a custom css plugin.
you will need to edit those styles:
/* One column */
.one-column #page {
max-width: 690px;
}
.one-column #content {
margin: 0 7.6%;
width: auto;
}
also, as Twenty Eleven is a 'responsive' theme, possibly change max-width to width; and reduce the margins of #content.
you will need to find the exact values yourself.
don't forget to clear the browser cache (click 'reload' or press 'CTRL F5') to check if the changes to the css were successful.
btw: this was previous asked and answered several times in this forum - forum search does often bring up solutions.