World’s largest postcard?

The biggest postcard in the world does not fit into any known terrestrial letterbox. It measures 24 metres by 1.35 metres and shows a 3700-kilometre long, 166-kilometre wide strip of Martian landscape in south-north direction.

Read more:

http://www.esa.int/SPECIALS/Mars_Express/SEMYD457ESD_0.html