Do you want the option so the users can adjust the width them self or you just want the template wider?
If you want to set the template wider you can do this:
Open up template_css.css on line 740(original file), if not do a search in your file for this:
#ja-wrapper {
margin:0pt auto;
width:750px;
}
There you can set the width to what you want the template to be.
The find this on line 749 in template_css.css:
#ja-mainbody {
float:right;
width:562px;
}
Change width: 562px; to the width of the mainbody you want, this will be a try and fail for you until you find out which width that fits
the width you set on the template.
Hovewer, you can change it to % instead of px if you want,but you still need to play around with it.
Give this a try and see if you can get the resoult you want, if not let us know?