Tutorial
The width of your site is can be updated for those using the 'Legacy' theme. If using a theme, the site width cannot be edited.
The site width can be managed under Appearance Master Template
Master Page Width
Enter a desired width into the 'Master Page Width' field found under the 'Master Template' section.
Click the button
Note
The header graphic on the site should be the same width as the 'Master Page Width' setting.