Architecture of database servers is one of the important parameters in the performance of web applications. In this paper, a model is proposed for guiding Postgre SQL database server sizing for concurrent users. Coloured Petri-nets are chosen to represent the model that brings out need to change the deployment architecture when the current architecture may not suffice. The focus of the proposed model is mainly on identifying how the increase in number of concurrent users and type of query affects the performance of the database server. The model is based on the experience as well as experimentation. The model is demonstrated in PlPE-2 using different scenarios. The paper also covers analysis of the proposed model.