Sign Character Height and Viewing Distance (2010 ADA Standards 703.5.5)
Letter height is not a design choice: Table 703.5.5 reads it off how high the characters sit and how close a person can actually get, adding 1/8 in for every foot of viewing distance past the band threshold. The band boundaries are cliffs, not ramps -- a baseline at 70 in needs 5/8 in characters and the same sign raised a tenth of an inch needs 2 in, because the base jumps bands. Two measurement rules do most of the damage: the height is to the BASELINE, not the panel, and the viewing distance runs to whatever OBSTRUCTION prevents further approach, so pulling out the counter in front of a sign makes the distance SHORTER, not longer.
Formula and source
required visual character height = base + (max(0, viewing distance - band threshold) / 12) x 1/8 in, where the band is set by the height of the character baseline above the finish floor: 40 to 70 in -> base 5/8 in, threshold 72 in; over 70 to 120 in -> base 2 in, threshold 180 in; over 120 in -> base 3 in, threshold 252 in (21 ft).
2010 ADA Standards for Accessible Design, 703.5.5 and Table 703.5.5 (Visual Character Height). A US federal standard in the public domain, quoted directly.
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