For iS101 and 60kg a general or typical range would be
40-42cm = marginal to light 7.3m conditions
34-36cm = powered 7.3 conditions/light 6.3 conditions
32-34cm = powered 6.3 conditions
30-32cm = overpowered 6.3 conditions
You don't need all these sizes, it's simply an indication of how the iS101 would be tuned for your weight and sail range. Of course, there is tolerance (at least +/- 2cm) that needs to be taken into consideration because of rider style/technique/strength, plus wind (solid vs gusty) / water conditions (flat vs choppy) and sailing program ; slalom ( Fig 8 or downwind?) or GPS speed or just blasting. And of course the actual fin make/model for a given cm size has influence on "performance" size.
It's not clear what your previous board was, but in general iS101 sets up best with a little more fin than narrower (or narrower tail) slalom or freeride boards of similar volume and range.
Cheers ~ Ian