There are no standard dimensions for a surfboard rocker (the stringer) since it all depends on the waves you surf and how you want the board to perform.
Stock surfboard blanks have proven rockers that are a great starting point and maybe a little adjustment is needed while shaping to get the desired rocker you want to surf.
Typically, if you are making your own stringer, you simply trace the blank foam outline on a piece of wood and cut out to match. Then glue it all together.
If you want some dimensions to start you can copy the rocker on our 6'3 Fish blank