with ImageEnMio, AcquireParams do
begin
  ScPSizeMax:=-1;
  for i:=0 to length(IOPDFPaperSizes)-1 do
    begin
      with IOPDFPaperSizes[i] do
      begin
        If (Width=PhysicalWidth) and(Height=PhysicalHeight) then
        begin
          case Size of
            iepA3 : ScPSizeMax:=TWSS_A3; *)
            iepA4 : ScPSizeMax:=TWSS_A4;
            iepA5 : ScPSizeMax:=TWSS_A5;
            iepA6 : ScPSizeMax:=TWSS_A6;
          end;
        end;
        if ScPSizeMax<>-1 then Break;
      end;
    end;
    if ScPSizeMax=-1 then ScPSizeMax:=TWSS_A4;
...

procedure TfrmMain.btnShowMaxSizeClick(Sender: TObject);
var
  i: Integer;
  maxPageSize: Integer;
begin
  maxPageSize := -1;
  for i := ord( iepA0 ) to ord( iepB5 ) do
  begin
    if ( IOPDFPaperSizes[ i ].Width <= Round(ImageEnMView1.MIO.AcquireParams.PhysicalWidth * 72 )) and
       ( IOPDFPaperSizes[ i ].Height <= Round(ImageEnMView1.MIO.AcquireParams.PhysicalHeight * 72 )) then
    begin
      maxPageSize := i;
      Break;
    end;
  end;
  if maxPageSize > -1  then
    ShowMessage( 'Max Metic Page Size: ' + IEPaperSizeToStr( TIOPDFPaperSize( maxPageSize )))
end;

with ImageEnMView1.MIO do
begin
  lblSizeM := 'Max Paper Size (Metric): + IEPaperSizeToStr( IECalcPaperSize( AcquireParams.PhysicalWidth, AcquireParams.PhysicalHeight, True ));
  lblSizeUS := 'Max Paper Size (US): + IEPaperSizeToStr( IECalcPaperSize( AcquireParams.PhysicalWidth, AcquireParams.PhysicalHeight, False ));
end;