Step size is calculated based on the selections of some parameters.
for example if nonlinearities are selected then, step size will relate to the max nonlinear phase shift entered by the user.
if no nonlinearities selected then, the step size is calculated based on the fiber total length and a stored number of sections in the fiber.
if there are many fiber sections, yes the step size would change from one fiber to another based on the options selected by the user.
Hope that helps,
Mohamed