Guilin Sun
Ansys Employee

For the monitor spsans, yours are ok. But you can use much larger size, since FDTD will only use what is needed.

As for the source, 250nm or half waveength both are ok. longer distance can have the source be placed in large mesh cell, which is more effective to suppress back propagation (the grey region's functionality).

For distance, you can use the longest wavelength as a reference. have wavelength is a rough estimation, since usually there is a dielectric with refractive index not 1.  Suppose the refrative index is 1.5, so half wavelength in vacuum is about one thirdth of the wavelength in material. For visible light, it is roughtly 250nm.