RE: Fin size to match 9.5 on iS133??
From Starboard's product site, it appears that the board comes with 2 fins. The larger of the two should be a 48cm fin, and I would tend to think that it should work well matched with a 9.5 sail. You could go a bit larger, say like a 50cm, if you goal is planing in the very lightest of winds, but its likely that this larger size would limit your speed potential as the wind picks up.
What I would recommend is experimenting a bit with the 48cm to see how well it works with the 9.5. I would try to discern whether you can readily feel the lift of fin while in the straps. Also, you might want to determine your potential for spinout. If you can spinout fairly easily pushing hard to windward, it's a sign that you could very likely benefit from a larger fin. Lastly, I found that if the sail feels like it lacks power and a lively character, moving to larger fin can often resolve this, as the fin and sail must provide a good balance together to be effective.