This is not actually an answer to the original poster, but once you use very short SUPs (less than 7'), you may want to choose your paddle length exclusively for the handling in the surf and forget the paddling efficiency, as these boards do not paddle well anyways...
A short paddle length then better fits in small to medium wave faces. For my 7'7" I use a paddle of my height +1" (I use +5" normally, and +9" on small cool waves).
At the extreme you can see the use of a paddle
minus 4" for a 6'3" SUP (paddle is 175cm for a 186cm rider)
See:
www.gong-galaxy.com/magazine/movies/tech-sup-shortboard/