The theme Twenty Eleven is an ancient theme designed for the predominant design trend of the era (eg fixed layout). And, back in 2011, themes did not come with the million-and-one customization options today’s themes come with. This is why you have very few options in Appearance > Customize.
My recommendation is to use a modern, ideally full-site editing, theme.
But if you choose to build your new site with such an old theme, you need to either settle for its limitations… or be willing to write custom code to achieve any modern enhancements you desire.
You may try the following custom CSS to reduce/remove the margins (put in APPEARANCE > CUSTOMIZE > ADDITIONAL CSS):
#page {
max-width: unset;
}
This will drastically reduce the margins on either side of the page. If you want to remove all the margins, add this as well:
body {
padding: 0;
}
With more horizontal space at your disposal now, you may want to increase the width of the content area and/or right sidebar. Let me know if this is of interest to you as well.
Good luck!