This image shows the relative sizes of two martian impact basins with a map of the United States for scale. Argyre Basin has a diameter of about 1100 kilometers. Hellas Basin (diameter 2300 kilometers) is the largest well-preserved basin on Mars. Other, larger basins are believed to exist on Mars in highly degraded form. Chryse and Utopia (sites of the two Viking landers) are believed to be examples of such old, degraded basins.
From Meszaros, 1985, Plate 60