Используйте этот код:
Dim redirect As String = "<script type='text/javascript'> window.open('PrintOrder.aspx?OrderNumber=" & Me.ViewState("OrderNumber") & "', '', 'left=100,top=100,height=350px,width=400px,status=yes,toolbar=no,menubar=no,location=no,scrollbars=yes', false); </script>";
ClientScript.RegisterStartupScript(Me.GetType, "openWindow", redirect);