Thanks to prolific poster DJ over at Seabreeze Forums!, we now have this visual approach to paddle length. Even DJ would agree, however, that a little shorter is a little better if you’re going to be using your SUP mainly in the surf. DJ says:
Because paddle blade size can vary and you want the blade fully dipped I like this system to find the right length for you.
DJ
Admin note: the illustration below comes from Starboard. 