We use a large sample of radio sources to investigate the effects of evolution, luminosity selection and radio source orientation in explaining the apparent deviation of observed angular size redshift (θ− z) relation of extragalactic radio sources (EGRSs) from the standard model. We have fitted the observed θ− z data with standard cosmological models based on a flat universe (Ω 0= 1). The size evolution of EGRSs has been described as luminosity, temporal and orientation-dependent in the form DP, z, φ≈ P±q (1+ z)− m sinφ, with q= 0.3, φ= 59◦, m=− 0.26 for radio galaxies and q=− 0.5, φ= 33◦, m= 3.1 for radio quasars respectively. Critical points of luminosity, log P crit= 26.33 WHz− 1 and log D c= 2.51 kpc (316.23 kpc) of the present sample of radio sources were also observed. All the results were found to be consistent with the popular quasar/galaxy unification scheme.