Wikipedia:WikiProject Geographical coordinates/dim:

The  parameter defines the diameter of a viewing circle centered on the coordinates. While the default unit of measurement is metres, the  suffix may be appended to indicate kilometres. The  parameter overrides the scale implied by any type: parameter.

Syntax — dim:&lt;width&gt;&lt;units&gt;
 * width — integer number of metres (m) or kilometres (km)
 * units —  or , if not specified, defaults to

GeoHack uses  to select a map scale such that the viewing circle appears roughly 10 cm in diameter on a 72 dpi computer monitor. If no,  , or   parameters are provided, GeoHack uses its default viewing circle of 30 km.