To determine the best roll-up banner font size, first figure out where the banner will be placed and how far away people are going to be when viewing the banner. The further away a person is, the larger the fonts should be so that they can easily read the banner.
Once you’ve determined the probable distance your viewers will be from the banner, a good rule of thumb is to make the smallest banner font size at least 72 pt for every 10 feet of viewing distance. You can further facilitate readability by using sans serifs fonts like Arial, Helvetica, etc. Avoid italics and serif fonts.