The width of your site is controlled within "Edit Master Template", within your Control Panel.
1. Under 'Master Settings' choose 'Edit Master Template'
2. The field that needs to be changed is 'Master Page Width' which you can increase or decrease to your liking.
If you do not see the immediate changes, you may need to clear your recent browsing history by choosing 'CTRL-R'
NOTE: The header graphic on the site should be the same width as the 'Master Page Width' setting.