That is set in Style & Templates > select your Style and go to All Style Options > Sizes and Dimensions > User Control Panel Message Area Width (pixels)
Quote:
This setting allows you to set the width of the message area in user control panel. The Send New Private Message form, for example (if you have the PM feature enabled).
Enter a number of pixels followed by a "px" (e.g., 380px).
03-15-2008, 08:41 PM
PM Input Field Width
Linear Mode
