After my research, I have ended up more confused than before I started.
Every body generally aggrees that a surf paddle should be shorter than a racing / cruising / down-wind paddle, but how short is too short?
From the videos and photos posted on here, there are obviously plenty of talented surfers on the 'breeze. I would love to hear what you are all using.
Surely the crew that compete against each other have noticed a general rule...?
Mine's 8" taller than me. I think some guys like them shorter for surfing - depends a bit on how bouyant your board is I'd imagine as you would be slightly higher above the water with a high volume SUP.
More leverage with a longer paddle.
8" to 6" above your head. Lean more towards 8" if you are going to do more flat water stuff or 6" for more surf stuff.
I've got 1 at 10" for racing, as my race board is thicker & 1 at 6" for surf stuff on my smaller sup.
Hope this helps
XX angie
Current paddle is as high as my reach above my head which is fine on my down windboard
Feels a bit long when surfing so I'll probably get another paddle at some point and run it 6-12" shorter currently I just grab my partners for surfing (don't tell her 
).
Cut your paddle to length then tape the handle on with silver plastic duct tape. Ride it for a few sessions then knock another inch off. Repeat until you are happy.
A good starting point is to stand with your arm above your head. Extended but not stretching. The handle should nestle nicely in the palm of your closed hand.
Progress slowly. Rule one in SUP is your skill level will change so what you thought when you were a noob will not apply after a few weeks/months of practice.
The tape will be waterproof and super strong. Start with a wrap or two around then a couple of figure-8 wraps around and over the handle. Don't try to get the tape super smooth. The wrinkles make a good texture and improve grip and give the tape a 3D element to make it stronger.
Hopefully you have a paddle with a T-grip handle. The ergonomic handles suck. They're too thick to fit into the palm of your hand.
6" to 8" is pretty common and seems pretty comfy. As angipangi said, closer to 8" if you do a bit of flatwater cruising as well as surfing and also if you are on a big thick board but closer to 6" if you really only surf and are perhaps on a smaller board that sits lower in the water. Play it safe and start a bit longer - you can always shorten it - but it's a bugger to add 2 inches if you go too short to start with 
hey Craigus,
i m 105kg 6ft 3 and use a 130ltr (approx) 10 4 board and i use a paddle 5inch over, it feels great in the waves , on my 12 fter i believe something around 6 - 8 over would be better obviiously to get further foward with the reach.
Thanks guys. I can deffinately afford to loose a couple of inches then.
Now - the hacksaw!
In canoeing (and OC paddling ) the best leverage position is when you pushing your top hand as parallel as possible to the water ...thats punching straight out at shoulder height
If your top hand is too high your reaching but pushing down on the water on the blade entry .... too low your either bent over to compensate or shortening your stroke and lifting water on exit of the blade .... and the leverage on the blade is less efficient ...
The variables are the blade shape and the thickness of your board So using this "Punch straight out theory" its hard to get that measurement just right ... But a way to find out just how close you should cut the little sucker is to measure from your eye level to the floor take this measurement and measure from the place where your paddle shaft hits the blade (and please including the grip in the measurement) add a few inches and cut the shaft .... tape it up ... try it out .... then think about it .. then do your final cut and glue .. Using the straight punch parallel to the water theory ... When you turn the paddle upside down your eye level should be just below the point where the efficient part of the paddle joins the shaft .. But then again all my paddles are a lot longer than that ... so go figure ... Surely the guys at the shop where you buy the paddle should have a demo .. if they dont your possibly shopping at the wrong place ....
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/
