RE: Isonic 122
It depends alot on how pwered up you are on a particular size rig, and whether or not you have steady consistent winds or gusty winds with holes.
I've sailed the Isonic 122 alot with weed fins (32 cm Tangent Reaper and 32 cm Lessacher Duo), and sails from 8.5 m2 NX slm down to 5.6 m2 (Sailworks Hucker) but I don't have all that much experience with the stock 380 and 440.
My educated guestimate would be (for going as fast as you can) the 380 up to 7.5-8.0 and the 440 fin for larger.
If you are racing slalom, where upwind is a bit more important, then use the 440 fin fro9m around 7.0+ and up. This will help with early planing, and getting going in lulls and give you a better overall VMG.
For pure speed sailing, or just back and forth blasting, use the smallest fin you can get going with.
Hope this helps,