You could try checking out:
http://www.vbulletin.com/forum/forum...er-size-issues
Altering the style to fluid rather than fixed makes it stretchable based on browser window size. I have no experience with this as I barely touch these fixed/fluid options. Not sure if it is the answer for you but you could give it a try.
I do believe that the:
doc_MaxWidth and doc_Width under Styles & Templates -> Style Manager -> "YOUR STYLE" -> Style Variable Editor should be set to auto for fluid.