I want to develop an SSRS report that, based on a parameter, will print conditionally to an A3 sheet or an A4 sheet. In other words if parameter value A is selected, set the report's PageSize Width property to 42cm and PageSize Height property to 29.7cm. If the parameter value B is selected, set the report's PageSize Width property to 29.7cm and PageSize Height property to 21cm. There's no option to use an expression for these properties.
Hmm..Nice one. But could you please specify why you need that? Since not sure if this is correct understanding but increasing the page height could be taken as increasing the number of records per page? And for expanding the width there could be a hidden/suppressed dummy field with expand property set to TRUE. When you need to expand, that dummy field could be filled with some data. I have not tested it due to time constraints, but seems like worth trying it if this is what originally required.