In the present paper, the modal optical fields within Photonic-Crystal Vertical-Cavity Surface-Emitting diode Lasers (PC VCSELs) are calculated with the aid of the Plane Wave Admittance Method, a new advanced threedimensional vectorial optical model. We determine the optimal diameter of PC holes within the PC VCSEL distributed-Bragg-reflector, which ensures the best field confinement within the VCSEL cavity. Alternatively, we also identify the critical diameter of the holes forming the photonic crystal, for which strong field leakage is observed.