The mesh delta is the spacing of the FDTD mesh used for the calculations. For the stability of the source it should be at most 1/10th of smallest wavelength in your source as measured in the highest refractive index in your design. The auto setting should ensure that for you. You also need to make sure that your features are properly discretized. In photonic crystal design it is common to have features that require a more dense (smaller delta) that what is required for the source. In designer check the “Refr_Idx-Re” tab and ensure your structure is acceptable.
Scott