How to set A4 size for FlowDocument under Window in WPF C# ?

Question

I have a flowdocument under a Window in WPF C# desktop application. I want to print the flow document for A4 size. How do I set size so that content under the flow document covers full A4 paper size? 

 

<Window x:Class="CarpetChallan.Window1"

   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"   

   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

   xmlns:d="http://schemas.microsoft.com/expression/blend/2008"

   xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

   xmlns:local="clr-namespace:CarpetChallan"

   mc:Ignorable="d"

   Title="Window1" Height="730" Width="800">

   <Grid>

      <FlowDocumentReader ScrollViewer.CanContentScroll=" ScrollViewer.HorizontalScrollBarVisibility="Auto" ScrollViewer.VerticalScrollBarVisibility="Auto" IsPageViewEnabled="True">

         <FlowDocument ColumnWidth="999999" PageHeight="21cm" Name="CallanFlowFoc">