As another helpful addition to our [Resolution vs Size] post, here’s a guide to deciding on minimum text height at varying viewing distances.
Often in our travels we notice billboards, car magnets, and other signs of all types that aren’t legible when passing by. If the text is too small, you can’t very well expect your customers to read it!
Australian Standard AS1319-1994 provides guidelines for symbol and text heights on safety signs. These are a great guide for legibility on all types of signage.
Pictograms: 15mm height per metre of viewing distance
Upper Case Text: 5mm letter height per metre of viewing distance
Lower Case Text: 4mm letter height per metre of viewing distance