mail preferences window now wider than screen

I was creating a new mail signature, and when I was done, the preference windows suddenly got wider. The mail preferences window is now nearly twice the width of the screen.

If I slowly drag the window over enough to see the lower right corner, I can not get the window to resize (tried with various optional keys with the mouse).

The green button on the top bar is gray. Generally the green button will enlarge or return to normal the size of the window.