Page Editor Width Spans entire window

For some reason TinyMCE is spanning the entire width of my browser window, but only when I’m editing/creating a page. Also the right edit sidebar floats over the top of the post box, obscuring the text and making it dificult to edit properly.

I don’t think I changed anything that would have caused it, and its not a browser issue, as i’ve tried it on multiple machines with different browsers. Is there any way for me to manually fix the width of the editor?

Click here for a picture of the problem.