Distraction Free Editor – adjust width

I’m using the latest version of WordPress [4.0] and trying to use the distraction free editor, but the DFE [once I go into it] is more narrow than the normal composition window.

As I’m typing, once I get to the far right side, the letters are disappearing for about six or eight characters; like they are behind the curtain. this occurs for the length of the article. Zooming in and out [CTRL + -] from the browser does not affect the problem with the right margin.

I need to be able to adjust the width of the DFE window. If someone could please tell me which CSS file controls this and how/what to modify, I would be very appreciative.

Later,
Larry