There are some general guidelines you can follow regarding
size of symbols on a map. The key is
legibility - that is, ability to be seen AND recognized. Legibility can be affected by: size of symbols and type, Contrasting colors and shapes, Familiarity, Perfect vision and perfect viewing conditions
In relation to size in particular, the ability of an object
to been seen from a distance relates to what is called the visual angle – or “a
size that subtends one minute of angle (1/60th of a degree) at the eye”. This is the angle between the light rays from the two ends of the viewed object as
they hit the eye, as shown in the figure below. It is usually measured in degrees. Read More...
