According to the (conflicting) documentation of TinyMCE, the editor takes on the size of the textarea (or other) element that it replaces. It also says that you can set the
I found the TinyMCE text-area (called '.mceContentBody' in my theme) too small and awkwardly margined in the new post content area. In my theme there's a css file called editor-style. I changed the width (to 98%) yet since TinyMCE uses cookies to remember the editor size, the CSS changes weren't sticking. After CLEARING MY BROWSER'S CACHE, the CSS width/margin changes took effect. Finally. Gah.