It really depends on the style your using, if your using a custom style then it could well mean you will need to edit the CSS, depending on how the designer made the template.
If your using the default style for instance then it can be as simple as going into your AdminCP
Styles & Templates->Style Manager for the style you want to widen select the
Stylevars option.
in the search box on the left type
doc_width, once highlighted select the units as
% and size
100 for full width, or choose a slightly lower number to suit your needs, then hit save.