Because of the flatter shape of the One versus many of the other SLE, Bow or Flat kites out there you can get a really good low end power out of our kites. As a result I would say, based on your weight and where you are riding, that the 12 would be ideal for you.
That being said, you could consider a 9 / 16 combo as well but I think the 12 would be your best bet for one One kite.
Line lengths with the kite come standard as 21m with 3m extensions. This is because the combination of the line leaders and the bridles on the kites add up to an additional 3m, making the line length overall to be 27m. Drop the 3m extensions to remove a bit of power but to also speed the kite’s turning up a bit.