ANSI Z535-2007 standard uses a ratio of 25 ft of viewing distance per inch of text for Favorable Reading Conditions, and a ratio of 12 ft of viewing distance per inch of text for Unfavorable Reading Conditions.
Determining the safe viewing distance of a sign depends upon many factors: lighting, background, type font and visual acuity. Other factors to consider in determining sign size include: the complexity of the message, the reaction time necessary and the angle the sign will be seen from. In many situations it is better to use multiple smaller signs that are strategically positioned than one large sign.
Many times letter size may need to be larger than the values shown on the graphic for a variety of reasons: