Making my theme thinner?

I want to make it thinner though, so that a background image can be seen a bit more. At the moment there’s only a bit of space either side, and the sidebar is much bigger than it needs to be (but I do still want to keep a sidebar). How can I make the actual page smaller so that more background shows/

Also I just wondered if it’s possible to make the pictures less ‘stretched out’ (the feature pictures which are on the home page)? They go wide and stretchy and look a bit strange.

My blog is https://www.beatnik-mag.com. Ignore the pictures and content – they’re all just testers!

Thanks for any help.

Grace