Choice of Letter and Number Sizes

The choice of letter and number sizes for the sails depends mainly on the size of your boat. This corresponds to the World Sailing (formerly ISAF) standards

    For boat sizes up to 350cm, the letter & number size is 230mm
    For boat sizes from 251cm to 850cm, the letter & number size is 300mm
    For boat sizes ranging from 851cm to 1100cm, the size of the letters & numbers is 380mm

The positioning of the country letters and numbers depends on the class rules, but in all cases, these identifiers will be more to starboard than to port. Please refer to your class rules for placement and spacing.

The proposed letters & numbers are usually in self-adhesive insignia. It is preferable to glue them on a new sail. In case you have to modify your number, degrease the sail properly before gluing new stickers.

https://www.h2o-sensations.com/en/113-letters-numbers-tell-tails