Wikiversity: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}} Lua error: callParserFunction: function "#coordinates" was not found.
Palace of Westminster 1:5,000 {{coord|51.5006|N|0.1246|W|scale:5000}} Lua error: callParserFunction: function "#coordinates" was not found.
City of Westminster 1:50,000 {{coord|51.501|N|0.125|W|scale:50000}} Lua error: callParserFunction: function "#coordinates" was not found.
Greater London 1:500,000 {{coord|51.50|N|0.12|W|scale:500000}} Lua error: callParserFunction: function "#coordinates" was not found.