As noted elsewhere on this website, "visual acuity" and "conspicuity" and "cone of vision" are very important for signs, because motorists must be able to detect signs, read them and then react to them in a few seconds. So how much does the visibility change when a sign directly faces the driver (perpendicular to the road) versus one that may be flat against a building fascia (parallel to the road)?
The Larson Transportation Institute at Penn State University studied this phenomenon in 2006 through a grant from the United States Sign Council. Much of the work involved a literature review of prior, related research.
Several factors impact the necessary minimum size for letters, so that they can be detected and read quickly enough for the driver to react:
Number of lanes of traffic
Distance from the road to the sign
The motorist's speed
The angle at which the sign can be read
USSC has published a chart that ranges from a sign that's 10 feet from the road when the motorist is in the curb lane, versus a five-lane road in which the sign is 400 feet from the road. In the first instance, the sign's letters need to be at least 4 inches tall. In the latter instance, the letters need to be a minimum of 90 feet tall.
This study, entitled "On-Premise Signs: Parallel Sign Legibility and Letter Heights, may be accessed at www.ussc.org.