Can some one please advise what size of twintip I should buy?!i have a 12m north rebel and weigh about 83kg.water will generally be flat/choppy.wanting to do free ride/free style.want the board to take me from beginner through to intermediate!cheers in advance
X ride by north
Crazy fly raptor and bulldozer
Fly board radical
Naish motion
Cabrinah tronic
Shinn red, ultrasonic, and monk
For chop you need some decent rocker and tip flex but this sacrifices big pop, you don't want a stiff board for chop but this will give you pop, the above boards have a mix some better than others, around 134x 41 or 42 dependant on wind strength averages,I've owned some of the above and trialed but for me the shinn range are hard to go past, use your pads and straps if you get to demo as you'll get a true feel of the board
I'm going to leave it to much more experienced guys than me to answer this, but I will say I've ridden only 4 twin tips and all of them were incredibly different. I won't tell you what they were because you don't need influence from a guy as new as me but when asking about size, hopefully the guys who answer will talk about characteristics as well.
1 of the twin tips I rode was so stiff and hard on my knees it wasn't funny
1 of them was soft and nice but doesn't turn very well
My favourite is a wave style twin tip and carves really well and doesn't jar my knees from being so hard.
All 4 were pretty close to the same size as well.
Long storey short hopefully the experienced guys comment on those board characteristics as well as board size.
Good luck
Like Daniel, I have not a lot of experience, but...
For the same style of ride, ok maybe a little more freestyle, I love the Nobile 50/fifty and I'm at lease with the 138x39.5 on a 12m EVO
Also, I'm "only" 75kilos, so there is a bigger size for this board.
It's just my "feedback", hopefully that will help.
I let the experts talking now... ;-)
I'd say go 136x41/42 plus minus 2 cm in length. That is assuming you ride straps. Higher rocker go slightly bigger, flat stiff board go shorter. Width has a much bigger impact than length so be aware of that too. Obviously this is assuming you use your 12m kite in conditions one would normally fly a 12 
There are heaps of allrounder boards out there. Just grab a mates board and/or demo and see what you feel comfy with.
