Increase width of posts?

I have searched the support forums but can’t seem to find a solution to this?

The white box is around 1000px from what I can see and the images and text within my posts is about half of this, around 500px.

I’ve looked on here about changing the .post-inner and .section-inner variables in the .css file but it doesn’t matter if I change these values (or even delete them!) the page structure and width stays the same?

Any help would be greatly appreciated!