Page width
The width of a page is determined by the design the site uses. Most designs have a fixed page width. This means that pages are the same width, regardless of the width of the visitor's browser window. Some designs instead resize to fit the entire browser. Each style has its merits. Its best to try several designs to see what might suit your needs.
Some third-party designs are available in wider widths than the designs that are included with Sandvox.