<WrapPanel HorizontalAlignment="Stretch">

            <TextBox x:Name="txt1" Height="30" Width="300" Margin="5" HorizontalAlignment="Stretch"></TextBox>

            <ComboBox x:Name="myCombobox" Width="120" Height="30" HorizontalAlignment="Stretch">

                <ComboBoxItem>10</ComboBoxItem>

                <ComboBoxItem>12</ComboBoxItem>

                <ComboBoxItem>14</ComboBoxItem>

                <ComboBoxItem>16</ComboBoxItem>

                <ComboBoxItem>18</ComboBoxItem>

            </ComboBox>

            <Button x:Name="btnConverter" Content="Converter" Width="240" Height="30" Margin="5" Click="btnConverter_Click"></Button>

            <RichTextBox x:Name="myRichTextBox" Height="180" Margin="5"></RichTextBox>

        </WrapPanel>

 private void btnConverter_Click(object sender, RoutedEventArgs e)

        {

            string firstTxt = txt1.Text;

            string fontSize = myCombobox.Text;



            Run r = new Run();

            r.FontSize = Convert.ToDouble(fontSize);

            r.Text = firstTxt;

            myRichTextBox.Document.Blocks.Add(new Paragraph(r));

        }