Contemporary architects can design buildings with complex geometry and construction more easily thanks to modern CAD tools such as BIM and other parametric design tools. Also, some material innovations, such as the introduction of ETFE cushions into architecture, make complex and designs with complex geometry more accessible to designers. In large spaces where day light intrusion is important ETFE cushions are a good alternative for glass. The construction of the cushions makes it possible to design curved organic shapes in a cost-effective way. However, simulating the acoustic environment in such spaces is challenging. Even though a CAD model is likely to be available, preparing a CAD model for geometrical acoustics software is a labourintensive manual task, time-savings can be made especially if only part of the model is of interest. In this paper we investigate a case study of a mall in Liège, Belgium. By comparing three model sizes, the possibility of only partly modelling this large space as a time-saving measure is investigated. The full model, a moderately truncated version and severely truncated version were compared in terms of their impact on the local acoustics in the area of interest. T30, STI, the spatial decay curve and G(average) were all compared using the same receiver positions. A parametric study shows that for modelling a specific part of such a large and long space, reducing the model size to the "medium" size has a negligible impact on any of the parameters. The strongly truncated model is found to be adequate for all parameters in the area near the source.