For all iSonics: Take the tail width (width one foot off (30,5cm)) and the longest fin is -4cm the shortest -8cm.
E.G. tail widht iS101 09 = 45,4cm: max is a 41/42cm fin and min a 37/38
use the shortes for small sails and the longest for your biggest sail :-)
For top speed purposes and deep downwind you can go down to -10cm but not more. Better to use than a smaller board.