How to get paper size if my printer is TMetafilePrinter?

Question

I try to get width and heigth of page of my TMetfile printer

var MFPrinter: TMetaFilePrinter;

MFPrinter := TMetaFilePrinter.Create(nil);

MFPrinter.BeginDoc
DC := MFPrinter.Canvas.Handle;    

W:=GetDeviceCaps(DC,HORZSIZE)
H:=GetDeviceCaps(DC,VERTSIZE);

But always I get these results: W=286 and H=503 inappreciable which paper size I use (A3,A4,A5) or page orientation.

I think these dimensions is my screen size instead paper size.