PRISMA - an acronym of Policy Research Instrument for Size-aspects in Macro-economic Analysis - is an economic macro-sector model. It has been designed in such a way that it produces results consistent with those produced by the current macro-sector model of CPB, Netherlands Bureau for Economic Policy Analysis. PRISMA is used for forecasting, scenario building and what-if analyses with respect to government policies and exogenous shocks. Its time horizon is 3-25 years. PRISMA consists of a kernel and a number of modules. Its kernel is documented in De Wit (2001). PRISMA's most important module is the size-class module. In this report the present version of this module is documented. In the kernel of PRISMA the business sector is subdivided into eighteen industries. In the size-class module the non-primary private industries (thirteen in total) are further subdivided into three size-classes: small (0-9 employees), medium-sized (10-99 employees), and large (100 or more employees) businesses. Hence, the following two types of model exercises become possible. Firstly, economic effects derived by PRISMA's kernel - for example when forecasting, building a scenario, or evaluating the consequences of changes in policy or the economic environment - can be 'translated' into the prospects for SMEs. Secondly, when circumstances for SMEs change differently compared to large businesses - for example due to a policy measure that focuses particularly on SMEs - the consequences can be evaluated by using the size-class module.