Wikipedia:WikiProject Geographical coordinates/scale:

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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