Википедија:WikiProject Geographical coordinates/scale:

The scale: parameter specifies the desired map scale as 1:N, overriding the scale implied by any type: parameter.

GeoHack uses scale: to select a map scale for a 72 dpi computer monitor. If no dim:, type:, or scale: parameters are provided, GeoHack uses its default scale of 1:300,000.

Samples
Subject Scale Markup Result
Big Ben 1:500 {{coord|51.500611|N|0.124611|W|scale:500}} 51° 30′ 02″ С; 0° 07′ 29″ З / 51.500611° С; 0.124611° З / 51.500611; -0.124611
Palace of Westminster 1:5,000 {{coord|51.5006|N|0.1246|W|scale:5000}} 51° 30′ 02″ С; 0° 07′ 29″ З / 51.5006° С; 0.1246° З / 51.5006; -0.1246
City of Westminster 1:50,000 {{coord|51.501|N|0.125|W|scale:50000}} 51° 30′ 04″ С; 0° 07′ 30″ З / 51.501° С; 0.125° З / 51.501; -0.125
Greater London 1:500,000 {{coord|51.50|N|0.12|W|scale:500000}} 51° 30′ С; 0° 07′ З / 51.50° С; 0.12° З / 51.50; -0.12