RS designer does not support  the setting of common page size such as A4,B5,Letter and so on.With Aspose.Pdf for Reporting Services, you can get it as the following example.

Parameter Name: PageSize

Date Type: String

Values supported: A0,A1,A2,A3,A4,A5,A6,B5,Letter,Legal,Ledger,P11x17

Example:

 <Render>

…

<Extension Name="APPDF" Type=" Aspose.Pdf.ReportingServices.Renderer,Aspose.Pdf.ReportingServices ">

<Configuration>

    < PageSize >A4</PageSize>

</Configuration>

</Extension>

</Render>