Here's how I see this paddle length thing.
Here's some pics that I took of myself some time ago when I used to make my own paddles.
BTW.. I'm 6'4" and in these pics I'm using a paddle that's 12" above my height.
At the start of the stroke where you are about to reach forward and dip the paddle your top arm elbow should not be above your shoulder.
For me to get maximum power I found the my top hand needs to be pretty close to directly in front of my face as I push forward with it.
At this point the lower arm swings through an ark being almost straight and in order for the paddle blade to be fully dipped and maintain an upright stance with a slight knee bend but straight back there is a very specific lower hand position.
Because the lower hand position is pretty much set because of my above reason and if you were to shorten your paddle the only thing that can happen is to lower your top hand down but doing this reduces the leverage that you have over the paddle.
This pic of Laird shows what I mean.. Distance X is set... and distance Y is also set to get max power.. and shortening your paddle to distance Z will mean less power. (IMO)
So.. How does Matty, Blane and many other top surf SUP'ers paddle with a paddle that's so short (about 4" above their height).. They must either paddle with less power or bend at the back.. plus their sinky short boards has some effect also.
Anyway.. that's enough of me crapping on..

DJ