Hybrid semiconductor-metallic nanostructures play important role in a wide range of applications, and are key components in photocatalysis. Here we reveal that the nature of a nano-junction formed between a semiconductor nanorod and metal nanoparticle is sensitive to the size of the metal component. This is reflected in the activity towards hydrogen production, emission quantum yields, and the efficiency of charge separation which is determined by transient absorption spectroscopy. A set of Ni decorated CdSe@CdS nanorods with different tip size were examined, and an optimal metal domain size of 5.2 nm was obtained. Remarkably, charge separation time constants were found to be non-variant with metal tip size. It is proposed that electron transfer mechanism encompasses two consecutive but separate processes: slow charge diffusion along the rod towards the interface, followed by fast electron transfer from the semiconductor into the metal phase. The first diffusion step dominates the time constant for the charge separation process and is not affected by the metal size. The efficiency of charge separation on the other hand was found to be sensitive to metal size. It is suggested that Coulomb blockade charging energy, and a size dependent Schottky barrier, contribute to the metal size effect on charge transfer probability across the semiconductor – metal nano junction. These two opposing trends result in an optimal metal size domain for the cocatalyst. This work is expected to benefit a broad range of applications utilizing semiconductor-metal nano-composites.