procedure TForm1.WebBrowser1NavigateComplete2(Sender: TObject; 
  const pDisp: IDispatch; var URL: OleVariant); 
begin 
  WebBrowser1.OleObject.Document.Body.Style.Zoom := 0.50; 
end; 

{A page must be already loaded into TWebBrowser} 
//.zoom:=0.25; //25% 
//.zoom:=0.5;  //50%
//.zoom:=1.5;  //100%
//.zoom:=2.0;  //200%
//.zoom:=5.0;  //500%
//.zoom:=10.0; //1000%