Within the default v7.3 design page width is controlled within the following css within your default.css file:

.wrapper {
	width: 100%;
	max-width: 70em;
	margin: auto;
	border: none;
	background: transparent;
}

But you cannot make changes to the default.css file. You can make changes to your custom template sets custom.css file to override those css. To change just the page width you only need to insert the following into your custom.css file to override just that setting:

.wrapper {
	max-width: 100em;
}

For detailed instructions on editing the custom.css file look to this support wiki page.