I'm assuming you meant what screen width?

If you are designing a fixed width site, then you still need to design it to fit an 800px wide screen. Don't forget to allow for the browser edges, scrollbars, etc., which means a realistic max width of about 750px;

If you are designing for a fluid layout, then the site still has to be usable at this width without causing a horizontal scrollbar.

You need to check it for widths between this and the potential max to ensure nothing odd happens.