How to calculate if flyback diode needs heatsink?
Essentially the diode will need to dissipate the inductive energy stored in the load, at 100 times per second.  You’ll need to establish the inductance of the load and can then determine the power rating for the diode.  If you don’t know what the load will be then you have a problem.  You might imagine that a motor could dump quite a significant amount of energy if it’s operating as a generator and being actively driven.