Complex emission spectra of europium doped rare earth calcium oxoborates, EuCa4O(BO3)3 - EuCOB, and GdCa4O(BO3)3: Eu3+- GdCOB: Eu3+, were finely analyzed for better understanding of some local perturbations detected in these solid media. Highlighting a “size effect” of dopant / matrix ions, the interaction between the host lattice and the embedded ion is demonstrated. The evolution of the crystal field strength of R3+ ions along the rare earth series is presented for C-type RE2O3 oxides. According to R3+ - RE3+ ionic radii difference (R3+ for a dopant ion and RE3+ for a matrix ion), two opposite standings are evidenced along a series. Effect of high concentration doping on spectral line broadening is illustrated following a half-height width of 2P1/2 level of Nd3+ ion in A-type La2O3 oxide.