How do I expand the width of the right column in the admin panels widget page?

Hi!

I use many text widgets in my theme that contain several lines of code and images. The narrow right column in the admin panel’s widget page (Appearances->widgets) doesn’t give me much space to work with. However, in tinkering in the admin panel’s css I wasn’t able to hit on the right markup to produce the desired effect. I also can’t seem to find any plugins that do the trick. Does anyone have a simple way of making that column and its contents wider?

Thanks!