It has previously been shown by the authors that the pre-chamber ignition technique operating with fuel-rich pre-chamber combustion strategy is a very effective means of extending the lean limit of combustion with excess air in heavy duty natural gas engines in order to improve indicated efficiency and reduce emissions. This article presents a study of the influence of pre-chamber volume and nozzle diameter on the resultant ignition characteristics. The two parameters varied are the ratio of pre-chamber volume to engine's clearance volume and the ratio of total area of connecting nozzle to the pre-chamber volume. Each parameter is varied in 3 steps hence forming a 3 by 3 test matrix. The experiments are performed on a single cylinder 2L engine fitted with a custom made pre-chamber capable of spark ignition, fuel injection and pressure measurement. The measured main and pre-chamber pressure data is then used to perform heat release analysis to understand combustion phenomenon in pre-chamber and the ignition and combustion of fuel-lean charge in main chamber that follows. Within the span of parameter variation, it has been observed that a larger pre-chamber provides higher ignition energy which results in shortened flame development angle and combustion duration. It is also observed that at a given pre-chamber volume, nozzle diameter mainly affects the combustion duration which may be due to difference in penetration depths of pre-chamber jets. The varied parameters seemed to have minor effect on NOx emissions.