I also agree with most of the answers above, but I would like to add that the wind conditions and your weight play also an important role.
My 2008 IS122 has only 114L of volume and my old 9.5 sail became rather heavy on it when there was not enough wind to easily plane, making the board going very deep in the water with my 87 kg.
The 2010 IS121 has 117L, I believe.
With enough wind to easily plane without pumping, a 9.X sail works great on this board.
IS87 / IS101 / RRD114 / US147