RE: Sails on the Isonic105
I used the 105 with GTX '05 and OD '06/'07. The OD really works great on this board. You of course have to care for your weight.
OD 7.5: 12-18 Kts
OD 6.5: 14-22 Kts
OD 5.5: 18-30 Kts
This is a rough estimations from my own experience and fellow surfers in relation to your own weight. I have surfed the board with GTX 6.0, 6.5, 7.0 and OD 7.5. 6.5 to 7.5 really are nice sizes and I guess up to 8.5 won't be a problem.