The size of the banner letters should be determined by the distance from which they will be viewed. As a rule of thumb, for every 10 feet of viewing distance, the letters on your banner should be at least 1 inch tall. So, if you expect your banner to be viewed from 100 feet away, the letters should be at least 10 inches tall. This ensures that the banner is easily readable and its message can be clearly understood by the audience. However, other factors such as color contrast, font type, and lighting conditions can also affect visibility, so these should be considered when designing your banner.