[Theme: Twenty Eleven] Expanding the body area

try to use a tool such as Firefox’ add-on Firebug https://getfirebug.com/ for these kind of formatting problems.
helping with fundamental formatting is not within the scope of this Word~Press forum

please do not edit Twenty Eleven directly, but create a chidl theme first, to work with. https://codex.www.ads-software.com/Child_Themes

once the child theme is active, look into style.css of the parent theme, there is a max-width: 1000px; in the style of the css id #page.

make the corresponding adjustments in style.css of the child theme.