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