Changing line 185 of your includes/templates/classic/css/stylesheet.css to have width:100% would cause your site to expand to full width. But this comes with two "BUT"s ...

Firstly, it's important to follow-up on the points that dbltoe made above, otherwise you changes may be lost when you upgrade.

Secondly, I recommend taking a look at the results on a big screen before getting too far into your design. Designing full width layouts that work on the increasingly wide range of screen resolutions is becoming increasingly difficult to do well.