I did try changing this line of code in the editor-style css file.
“#tinymce {line-height:1.75em; font-size:18px; width:750px;}
#tinymce h1″
Putting in a new larger value for the width didn’t make the page posts any wider.
Note: Not a coding expert, just looked through the code for width of 750px to change. Sometimes that works, sometimes it doesn’t.