How wide is Mercury's largest crater?

It measures 1,400 kilometers across (roughly 875 miles). Its called Caloris Planitia but a better name might have been Deep Impact.

The crater was probably created by the impact of an object 100 kilometers (62.1 Miles) or more in diameter. It is believed that the energy from the impact was so powerful and focused that the traveled through Mercury and created 500,000 square kilometers (310,686 square miles) of disrupted terrain on the opposite side.

This false color image was taken by the NASA Messenger space craft in January 2008. The large orange area is the Caloris Planitia impact basin. The false colors in this image help scientist see geological features and mineral composition. For example the very bright orange spots near the edge of the basin are believed to show the location of volcanic features.