Simulation modelling is the business of reducing complexity of the real world. Keep your models small and simple, is the basic heuristic found in simulation & modelling literature; yet the modellers are developing models of increasing size and complexity. Simulation model size and complexity is generally a less debated and poorly understood area. Despite the importance of model simplification and its relation with size and complexity; only a handful of papers, addressing the size and complexity of simulation models, can be found. This paper reports on general concepts of simulation model size and complexity held by expert modellers and proposes a framework to understand the origins of simulation model size and complexity. Size and complexity are two most important contextual factors that may affect simulation practice. Increased understanding of concepts of simulation model size and complexity held by the modellers will potentially underpin simulation methodology research and help us in addressing the issues raised by increasing size and complexity of simulation models.