One of the biggest sign mistakes is using lettering that is far too small. There are three primary reasons people make this mistake.
With a standard inkjet or laser printer and letter size paper, you can properly size fonts for most signs.
If you think 4” letters will be big enough for your sign. Print out ‘Testing’ with 4” tall letters. Tape the sheets of letters together and put them where the sign will be. Have various people can read the test word from the spot where shoppers will first see it. Based on their feedback, you will know if you need to increase the size of the letters.
Also, font size required for legibility varies based on font style. A light-weight ‘scripty’ font (which I wouldn’t recommend in most situations) must be much larger to be easily read as compared to a stronger, simple font.