Set font size for FlowDocument
<Window x:Class="WpfApplication1.Window1" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="FlowDocumentPage" Height="450" Width="500"> <DockPanel> <Button Padding="20" Margin="40"> <FlowDocumentReader> <FlowDocument FontSize="12" xml:space="preserve"> <Paragraph TextAlignment="Center" FontSize="18"><Bold>bold</Bold></Paragraph> <Paragraph>Para<Italic>italic</Italic>HTML</Paragraph> <Paragraph>aaa<Button>Push me</Button> <RadioButton IsChecked="True">Yes</RadioButton> <RadioButton>No</RadioButton> <RadioButton>Unsure</RadioButton> </Paragraph> <Paragraph><Bold>Lists:</Bold></Paragraph> <List> <ListItem><Paragraph>A</Paragraph></ListItem> <ListItem><Paragraph>B</Paragraph></ListItem> <ListItem><Paragraph>C<Ellipse Fill="Red" Width="20" Height="20"/></Paragraph></ListItem> <ListItem><Paragraph>D<Rectangle Fill="Blue" Width="20" Height="20"></Rectangle></Paragraph></ListItem> </List> </FlowDocument> </FlowDocumentReader> </Button> </DockPanel> </Window>