public class PMPageSizeRestriction : PMRestriction

try
{
    //Get a reference to the printer object
    PMPrinter printer = PMPrinter.GetDirectPrintQueue("Server", "PrinterName");


    //Get all sizes
    List<PMPageSize> allSizes = PMPageSize.AvailablePageSizes();


    //Create a list including letter and A4
    List<PMPageSize> allowedSizes = new List<PMPageSize>();
    allowedSizes.Add(allSizes.Find(size => size.Name == "Letter"));
    allowedSizes.Add(allSizes.Find(size => size.Name == "A4"));


    //Set allowedSizes as whitelist, all other sizes will not be allowed
    printer.PageSizeRestriction.Enable(allowedSizes, PMRestriction.ListType.WhiteList);


}
catch
{
    //Error handling - See PM90SettingsAPI.Exceptions
}