[Theme: Twentyfifteen] Adjusting body content

Hi everyone, I am trying to make some changes to twentyfifteen (still prefer it over twentysixteen). I successfully got the left sidebar menu to shrink, but I would really like to try increasing the body content and center aligning it. Ideally, I would like to have something where the page’s featured image could be a header of about 1260×500 or 1260 x700.

You can see my site now at theredking.luchacomics.com

Any help would he greatly appreciated!