Text and graphics should be in relevant proportion to the environment they are in, so as not to be difficult to read from a normal and safe viewing distance.
AS 1319 Standard sets out the need to consider the lighting, the viewing distance, and the environment the sign is to be placed in.
The standard sets as a minimum that for a well lit, open work environment sign design needs to be:
Upper case text should be 5mm per metre of viewing distance
Lower case 4mm per metre of viewing distance
Pictrogram in proportion at a minimum of 15mm per metre of viewing distance.
Below is a quick reference guide to show how large the sign needs to be for the distance is needs to be viewed from. A great guide to take into consideration if you need to place signs around a perimeter fence.