Work supported by Ministry of Science and Education (MEC) of Spain (Refs. TEC2004- 03545, TEC2005-05781-C03-01, NAN2004-08843-C05-04, NAN2004-09109-C04-01), and contracts S-505/ESP/000200, UE NoEs SANDIE (NMP4-CT-2004-500101) and PHORE- MOST (IST-2-511616-NOE). The authors acknowledge useful discussions with Andreas H°akansson, Javier Martín and Daniel Torrent. Yu. A. K. acknowledges a support from MEC (Grant SAB2004-0166). A. R. A. thanks a FPU fellowship (Ref. AP2002-0474) and L. J. M. an I3P fellowship.We present theoretical and experimental studies of the size and thickness dependencies of the optical emission spectra from microcavities with hexagonal shape in films of two-dimensional photonic crystal. A semiclassical plane-wave model, which takes into account the electrodynamic properties of quasi-2D planar photonic microcavity, is developed to predict the eigenfrequencies of the confined photon modes as a function of both the hexagon-cavity size and the film thickness. Modes with two different symmetries, triangular and hexagonal, are critically analyzed. It is shown that the model of confined photon modes with triangular symmetry gives a better agreement between the predicted eigenmodes and the observed resonances.Work supported by Ministry of Science and Education (MEC) of Spain (Refs. TEC2004- 03545, TEC2005-05781-C03-01, NAN2004-08843-C05-04, NAN2004-09109-C04-01), and contracts S-505/ESP/000200, UE NoEs SANDIE (NMP4-CT-2004-500101) and PHORE- MOST (IST-2-511616-NOE). The authors acknowledge useful discussions with Andreas H°akansson, Javier Mart´ı and Daniel Torrent. Yu. A. K. acknowledges a support from MEC (Grant SAB2004-0166). A. R. A. thanks a FPU fellowship (Ref. AP2002-0474) and L. J. M. an I3P fellowship.Peer reviewed