Use the FontSizeConverter class to convert the content of a ListBoxItem to a value that represents the size of a font.

   
  


<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="FontSizeConverter_Csharp.Window1"
    Title="FontSizeConverter Sample">
<StackPanel>
        <TextBlock Name="text1" TextWrapping="Wrap" Height="300" Width="500">
            this is a test
        </TextBlock>
    <ListBox Grid.Column="1" Grid.Row="1" VerticalAlignment="Top" Height="50" Margin="10,0,0,0" SelectionChanged="changeSize">
        <ListBoxItem>5</ListBoxItem>
        <ListBoxItem>8</ListBoxItem>
        <ListBoxItem>11</ListBoxItem>
        <ListBoxItem>14</ListBoxItem>
        <ListBoxItem>17</ListBoxItem>
        <ListBoxItem>20</ListBoxItem>
        <ListBoxItem>23</ListBoxItem>
        <ListBoxItem>26</ListBoxItem>
        <ListBoxItem>29</ListBoxItem>
        <ListBoxItem>32</ListBoxItem>
    </ListBox>
    <ListBox Grid.Column="3" Grid.Row="1" VerticalAlignment="Top" Height="50" Margin="10,0,0,0" SelectionChanged="changeFamily">
      <ListBoxItem>Arial</ListBoxItem>
      <ListBoxItem>Courier New</ListBoxItem>
      <ListBoxItem>Tahoma</ListBoxItem>
      <ListBoxItem>Times New Roman</ListBoxItem>
      <ListBoxItem>Verdana</ListBoxItem>
    </ListBox>      
   
  </StackPanel>        
</Window>
//File:Window.xaml.cs


using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Navigation;
using System.Windows.Media;

namespace FontSizeConverter_Csharp
{
  public partial class Window1 : Window
  {
        public void changeSize(object sender, SelectionChangedEventArgs args)
        {
      ListBoxItem li = ((sender as ListBox).SelectedItem as ListBoxItem);
      FontSizeConverter myFontSizeConverter = new FontSizeConverter();
      text1.FontSize = (Double)myFontSizeConverter.ConvertFromString(li.Content.ToString());
        }

    public void changeFamily(object sender, SelectionChangedEventArgs args)
    {
      ListBoxItem li2 = ((sender as ListBox).SelectedItem as ListBoxItem);
            text1.FontFamily = new System.Windows.Media.FontFamily(li2.Content.ToString());
        }
  }
}