Depends on the power of your waves.
Basically, if you feel your leash is stretching a lot (it continues to stretch after the inital pull) , you should get a thicker one.
But my personal opinion is that it depends on the leash construction. A lot of leashes will fail at the junction, meaning that the leash did not benefit from the full strength of the cord. I used Creature of Leisure ones for instance, which has a well designed connection with the cord, but now only use the O&E ONE, XT if possible. A well designed leash will fail at the cord on the test below, meaning that the connections design was stronger than the cord.
In a nutshell: get an O&E ONE leash (or something as strong), and sand the trailing edge of your fins to avoid cutting it.