internal enum fModes

    Friend Enum fModes

static extern int DocumentProperties(IntPtr hWnd, IntPtr hPrinter, string pDeviceName, IntPtr pDevModeOutput, IntPtr pDevModeInput, fModes fMode);

For the values for the fMode parameter have a look at the fModes enum.

   int sizeNeeded = DocumentProperties(IntPtr.Zero, printerHandle, printerName, IntPtr.Zero, IntPtr.Zero, fModes.DM_SIZEOF);

   int result = DocumentProperties(IntPtr.Zero, printerHandle, printerName, pdevmode, IntPtr.Zero, fModes.DM_OUT_BUFFER);

EDIT 13.09.2013: For the values for the fMode parameter have a look at the fModes enum.