Changing the page width and featured image size

I’m using WP3.7.

At the moment, as you can see, there’s a large gap to the right of the body text and I’d like the page content to stretch across the entire page. I’d also like the featured image to stretch across the entire page but I’m not sure how to make it do this?

Lastly, I would like the page text to be justified, and not left aligned, but even Firebug doesn’t seem to be able to show me where the code is for this.

My site is: http://www.maxiselfstore.com

Any help would be much appreciated as I am very new to stylesheets – I’m using Firebug which does make life a little easier.

Thanks in advance for anyone who can help!