There is no optimal value for Java heap space, as the best allocation depends on what you are doing. The default heap size is sufficient in most cases.
Although, creating a larger Java heap gives more space to some types of objects (especially graphics objects or Simulink), it reduces the amount of memory available on your system for other tasks (like most MATLAB computations, and storing variables). If you are using a lot of Simulink and not performing very many calculations, then you will benefit from more Java heap space. If you are executing numerically intensive scripts with lots of large variables and do little or no plotting, then you may benefit from less Java heap space.