You can do this yourself
In admincp go to: Styles & Templates > Style Manager > All Style Options (Go)
On that page look for: "Main Table Width (pixels or percentage)" and change "100%" to 760 or 950 or whatever you like.
If you want to have both fixed and liquid you can just make a child style and do the above on the child. Here's an example of the 950px:
http://www.dutchbodybuilding.com/tes....php?styleid=4