It depends on a number of factors that you haven't specified. (Font, color contrast, illumination, vision acuity of intended audience, where the sign is going to be displayed). But if we are to assume a vision level of 20/40 (generally accepted standard for drivers licensing and thus of the public at large), and that the sign is manufactured and displayed in the optimal fonts and colors, at the distance specified, the letters would need to be almost 22 feet (actually 262 inches) in height to be viewable (with >75% legibility) if the sign is parallel to the viewer.
Of course, I may have missed something in doing my math.
BTW, I was using 28.6 feet/inch letter height (taken from the technical report within the UDA model sign code that ISA will be distributing later this year).