Dispersion Diagram PML problem

please , what are the hights of the radiation airbox and PML airbox to detrmine DISPERSION DIAGRAM

Assuming a planar structure (such as the Sievenpiper mushroom structure), with a conductor backing you will indeed want a PML above to simulate an open boundary. I've found you don't want it's distance above the structure to be too large. I usually go with roughly three times the largest period of the structure.