The ground state of the half doped manganite La_0.5Ca_0.5MnO₃ has a complex behavior, in which an antiferromagnetic charge ordered (AFM-CO) phase and a ferromagnetic (FM) phase can coexist. A systematic study of phase separation effects in polycrystalline La_0.5Ca_0.5MnO₃ under different thermal treatments is reported. Samples with average grain size ranging from 200 to 1300 nm were studied. Magnetic and electrical measurements show quantitative differences among samples in their low temperature behavior indicating that the fraction of the FM phase gradually decreases as the grain size increase. Percolation of the FM phase in samples with even a small fraction of this phase suggest that grain boundaries play a disctintive role in the spatial distribution of coexisting phases. The defective structure at the grain surface could explain the local inhibition of the AFMCO phase, an effect that is gradually removed by grain size increase. Qualitative agreement of the data with this description is found. Besides, this effect is also found to be highly dependent on the oxygen content and its spatial distribution.