Gotto throw my 2c in on the Twin tippers debate.
What I recon works from 10 to 50 knots.
Measure from the floor to your arm pit. That's your board length.
For the width measure your weight,
60 - 70 kg 36cm wide
80 - 90 kg 40cm wide
100kg+ 42cm+ wide
Jump off a chair a few times with your eyes closed and measure your stance.
Or set straps so there's around 30 cm behind your back foot.
Chop, More rocker, more out line shape.
Flat water, Less rocker, more square shape.
www.cardboarding.com has the answers for graphics.
www.newkiteboarding.com