For readability, plan an absolute minimum of 1 1/2 inches per 50 feet of distance. You should also be certain of any minimum size or contrast requirement imposed by your local authorities. Many of our address plaques go to homes that also have an additional, official, fire or police number posted. So size would not an issue in these cases. In general, address numbers of 3 inches is a good scale. On a larger building, larger plaques will have a better scale and look.