using System.Drawing.Printing.PrintDocument

using System.Drawing.Printing.PrinterSettings.PaperSizeCollection

using System.Drawing.Printing.PaperSize

using System.Drawing.Printing.PageSettings

/////////需要以上引用

PrintDocument pd = new PrintDocument();

PaperSize p=null;

foreach(PaperSize ps in pd.PrinterSettings.PaperSizes)

{   if(ps.PaperName.Equals("A3"))

  p = ps;

}

pd.DefaultPageSettings.PaperSize = p;

pd.Print();