You do not need to inter the exact number. Just keep the default value that I guess polystyrene. then use Di0.5 to express your particle size. Di0.5 is based on the intensity not the volume and represent 50% of the nanoparticles less than the value based on the intensity of scattered light. it is not appear by default in the software so you go to the setting and add it. for zeta potential also just keep the default value. more important is the temperature and the viscosity you must know if you are not using water.