colas said..It depends on your board & style.
A rule of thumb is: if you are on a board where you take off with parallel feet, DJ suggestion is sound. If you take off in fencing / surf stance, you will love a paddle your height.
But the real issues is the radius of your turns. The shorter you want to turn, the shorter your paddle must be, it can be down to -4" for performance in hollow waves.

Style is the key here. I am seeing a lot of guys going shorter especially out in the surf and you can see that they are adopting a paddling style where the
emphasis is on a quite pronounced bend at the waist which results in greater reach. This works for a smaller shaft length and in fact you could not yield positive
outcomes with a longer paddle as the paddle would be well below the water line. Think about how a longer paddle would effect cadence and you will start to understand
the reasoning behind this. You have a maximum height with a parallel stance but as you widen the legs going down the board then your height reduces resulting in the
requirement for a shorter paddle.
Cheers ozzie