Hi Jessica,
I had the same issue, I modified the .php files. You want to make the change in two places if you want the width to expand for both posts and pages.
For the pages, go to Appearance > Editor > Page Template
Replace this:
<div class=”pagewrapper fullwidth pagecenter withsidebar”>
With:
<div class=”pagewrapper widewidth pageleft withsidebar”>
Also, for the “Sidebar” in the Editor, I removed all the code completely. Just be sure to keep a copy of all original code for those pages in case you need to restore them.
CarmenLeahMcDonald.com