We present the results of measurements of the electrical resistivity (), magnetoresistance (MR) and dc and ac susceptibility () of polycrystalline samples of the type La_0.5Ca_0.5-xBa_xMnO₃, synthesized under identical heat-treatment conditions. The substitution of larger Ba ions for Ca results in a non-monotonic variation of the Curie temperature (T_C) as the system evolves from a charge-ordered insulating state for x = 0.0 to a ferromagnetic metallic state for x = 0.5. An intermediate composition, that with x = 0.1, interestingly exhibits ferromagnetic, insulating behaviour with thermal hysteresis in the ac at around the Curie temperature (T_C = 120 K). The x = 0.2 and 0.3 compounds exhibit semiconducting-like behaviour as the temperature is lowered below 300 K, with a broad peak in at around 80-100 K; these compositions exhibit a weak increase in as the temperature is lowered below 30 K, indicative of electron localization effects; these compositions also undergo ferromagnetic transitions below about 200 and 235 K respectively, though these are non-hysteretic; most significantly, for these compositions, the MR is large and conveniently measurable over the entire temperature range of measurement below T_C and this experimental finding may be of interest from the applications point of view. We infer that the A-site ionic-size mismatch plays a crucial role in determining these properties.