Your header should at least be as wide as the page's grid, covering the area where the main content resides. However, it's completely acceptable for the header to extend across the full width of the page.

When designing a full-width header, include sufficient padding around the edges. This padding prevents elements like logos, navigation links, or buttons from being right against the edge of the page, which can make the design look cramped and can cause usability issues, especially on touch devices where users might accidentally press the wrong thing.