Edit Post Page

I just upgraded to Twenty Fourteen. I used the CSS box to widen my posts from the standard 494px to 650px that I had in Twenty Ten.
The problem is, the editing box doesn’t know that I am posting in a wider field, so the visual is all off, both in terms of line breaks and images, from what actually posts. I’m a visual person so this is a bit painful. How do I (can I) tell the editor that we have a wider field to play in? Or is that just a fact of life I will have to learn to get used to?